catalina does not complete shutdown

Force quite by holding down the Power key for 6+ seconds. Then boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and reboot normally.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

I figured out the cause in my case. It was external drive that was hooked up to Time Machine backups. It was hanging. I just unplugged the device and my Mac shut down as normal. Plugged it back in on reboot and the problem was gone. It's been shutting down normally now for the past ten days.

I have had the same problem as described: Macos Catalina 10.15.1, 2 external monitors on 27" Imac, Catalina is on an external USB3 boot SSD drive. I tried everything mentioned including the SCM reset. Nothing worked, regularly had the system crash on shutdown. Then I remembered that the external USB boot drive was on a powered USB3 hub and thought. I wonder if having power on the USB was causing the SCM reset to fail. All of the crash logs seemed to be indicating an SCM failure so the reset should fix it. Anyway I did the SCM reset again but this time I unplugged the USB hub as well. This time the fix seemed to stick and so far my shutdown crashes have subsided. I know I just jinxed it but maybe it's a real fix, we shall see. Now if I could just get the system to remember my display brightness settings for my Imac. Have to reset that every time... No wait I had assumed it was wrong but that stuck as well. Looks like a solid fix all around...life is good.

I was having same problem - no external HD so that wasn’t the problem - I did have Microsoft Office 2008 and Photoshop CS5, both 32 bit, no longer supported And once I used Adobe cleaner to remove CS5 and then removed Microsoft Word my Catalina is working as it should and shut down is painless.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the app's Privacy preference pane so that it can get additional information from the Console and log files for the report:

ALSO IMPORTANT:

Then we can examine the report and see if we can determine the root of the problem.

I have noticed this after the supplemental update 10.15.4 on 4/8/20. It seems that either Finder, Activity Monitor, Safari, and in my case, also McAffee Internet Security/Total Protection have all been noted to be "Not Responding".

The only way I was able to shut down my 2018 15"MBP i9 was to force power down by holding Power button down.

My solution that I found so far- albeit only a few hours ago- power down. Power on and boot into Recovery Mode: Command + R. You may need to enter Firmware password if enabled.

Launch Disk Utility. Run First Aid on all your partitions. For me: MacOS, MacOS data, and boot up (don't recall the exact wording). MacOS and boot up partitions were ok. The data one found several mis-matched data to my 16 Time Machine snapshots.

After fixing them, exit and reboot.

So far- so good. "knock on wood"/

The first thing to do is uninstall all MacAfee software according to the developer's instructions That software is totally unnecessary for Macs and are known to compromise Mac and application performance. There is no need to ever run any "cleaning", "optimizing", "speed-up" or anti-virus apps.

This user tips describes what you need to know to protect your Mac. Effective defenses against malware and other threats - Apple Community   

Also it wouldn't hurt to download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the app's Privacy preference pane so that it can get additional information from the Console and log files for the report:

ALSO IMPORTANT:

Then we can examine the report and see if we can determine if there any other bad actors installed that should be removed.

I had same or similar issue. iMac would not shutdown just reboot and give crash notice. I unplugged an OWC Thuderbolt 2 dock attached to the iMac. Problem went away. I can shutdown iMac now. Heck it got rid of the the overnight reboots. Not sure if dock is bad, OS is bad or both.

What external devices are connected to the iMac ? I literally had a Thunderbolt 2 dock connected to my iMac. Once I disconnected it would shutdown. I moved the devices from the TB2 dock to a USB hub and it still works. Rather it shuts down properly. It may be an external device that is causing the issue. Or the OS. In fact the random overnight crashes have stopped as well. Probably just jinxed myself.

Did you migrate your home folder to your new Catalina installation? If so log into another user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

.. finally they solved the issue

i've updated to Catalina 10.15.4 and having no more trouble

system shut-off and reboot properly !!

FWIW - I had a very similar problem (with Time Machine backups on an external USB disk).

For me, the Dock was still active. So I opened a Terminal window from the Dock and in it typed sudo reboot, which resulted in a completed restart. Ever since then when I Restart... from the Apple menu my iMac restarts fine every time now.

.... Well... It seems Mac OS X and Chrome don't play well together. On every restart I get a message that the iMac can't restart because Chrome didn't respond, even though Chrome does close. I just choose Try Again and the iMac completes the restart.

Just a follow up on what I said about fixing the shutdown problem. This SCM fix only stuck for a day or two. After much searching and testing I decided to re-build my system from the ground up w/o migrating old data from my old system. It was time. I have been updating since 10.1 I think it was. Long time over many very stable updates this was the first time I had to do a full re-build from scratch. I figured all of the artifacts from old 32 bit software were getting in the way. Anyway this seems to have fixed the shut down problem permanently. It did not however fix the problem of the display brightness settings in my Imac re-setting to max after boot/reboot. FWIW this is a on a late 2015 27" imac with two 27" Apple external monitors. I'm sure this has something to do with the external monitors but who knows. Only the Imac fails to retain the brightness settings. Historically this has been a bug that crops up from time to time with new OS releases, all though I've never seen it on this Imac. I suspect it too will be squashed in the next release or two. Hope this helps someone else fix this problem.

On my mba 2019 it works fine, on my Mac Pro 3.1 (installed with dosdude patch) I got this problem... I thought it was because my system wasn’t fully supported! Anyway, whenever I want to shutdown I’ll run from terminal “shutdown -h now” and it goes.

When you rebooted normally after the Safe Mode did your Mac hang on shutting down?

If it did log into another user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).