macOS Catalina 10.15.5 won't update on my 13" MacBook Air 2015

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and try to download and install the macOS Catalina 10.15.5 Combo Update  from there.

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

I had this problems for months with the same laptop, so that I couldn't install 10.15.5 and then I couldn't install 10.15.6.

I noticed there were several downloads in system files for both updates that weighted around 2 GB each and I could not erase, so it appeared the OS itself was downloading them (probably incomplete downloads) and failed several times to install.

These are the exact steps that worked for me:

• Release as much space as I could in my disk (around 25-30 GB).
• Restart on Safe Mode, download and install the 10.15.5 Combo Update from the website.
• Wait like 1 hour for it to finish (screen time said 20 min).
• Restart on Safe Mode, download and install the 10.15.6 Combo Update from the website.
• Wait like 1 hour for it to finish (screen time said 23 min).
• Restart on Safe Mode, and then restart normally again.
• After the automatic cache clean up from restarting on Safe Mode, I had like 40 GB of free space because of the original downloads I could not erase, were flushed by the system. This way I could restore my documents and keep using my laptop as usual.

13 GB wasn't enough free space to allow the install. I tried again once I had 73 GB free. Maybe that's what was required, or maybe they updated the installer version by the time I gave it one more go.

I faced the same problem and the Safe Boot + Combo Update trick didn't help.

Check if you don't have a too much "OS-intrusive" software.

In my case, I had to uninstall Cisco AnyConnect. Update worked fine then even from System Preferences.

Resolved the installer problem by temporarily trashing the antivirus software. 10.15.5 installed with no problems. Retrieved antivirus and all is as well as could be expected.

@Random4837 was correct. Safe boot doesn’t work. You need to free up some space on your mac. I had 14 free GB. I freed it up to 17 GB and it is currently updating!

I was facing the same problem for the last 3 days since this update came.

1) I downloaded the O/S update and it was showing 2.98 gb and it was showing That I still have 8 to 9 gb of space left in my MAC.

2) As I pushed the update button the screen would blackout and then apple logo and restart.

3) on restarting I found its still not installed. I repeated the process several times but the results were same.

Here is what I did to successfully install it.

1) I deleted some old movies and videos to create a space of around 15 gb on my MAC.

2) Restarted and then started the installation process.

3) It took around 20 mins to install this new update. and screen going as Macintosh update will take nearly 37 mins, 9 mins more and so on. Until it installed fully.

So try it. I just explained the steps so that you don't get frustrated with the time it shows for installing an update.

Check if you don't have a too much "OS-intrusive" software.

I had to uninstall Cisco AnyConnect. Update worked fine then even from System Preferences.

Sorry for the double post, don't know how to delete previous one :(

Thanks a lot! I deleted my anti-virus software, and it finally works

This solution worked for me. I had only 11 GB free. After deleting some files, I managed to free up 15GB more. Installation was completed without any problems.

(MacBook Pro 13" 2014)

Had the same problem with about 16GB left on the SSD. Clearing up some of that space did the trick. Don't know how much would be required. I now have 95GB free.

I had this issue but have resolved it by freeing up some disk space. Upgrade then proceeded without any further issue.

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and try to install from there.

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