MacOS Catalina - Won't download from App Store

Did you try running First Aid? You can also try upgrading while in Safe Mode:

About Using Safe Mode

• Use safe mode to isolate issues with your Mac - Apple Support
• Playing Safe- what does Safe mode do?

Boot Into Safe Mode

1. If your Mac isn’t already shut down, then shut it down and wait at least 30 seconds before rebooting.
2. Immediately, at or before the chime, press and hold down the SHIFT key. 
3. Release the key after the Apple logo and progress bar appear.
4. Safe mode startup is much slower than normal startup, so be patient.
5. When the Login Screen appears enter your admin password. If you use automatic login, then this means you are in safe mode.

While in safe mode try upgrading. Be sure that you delete any prior downloads of the same installer before re-downloading. You may just have a slightly messed up installer download.

At this point I would guess there is some other weird problem at work. The following is tedious but may help.

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences, if needed, after resetting the PRAM.
6. Test in safe mode to see if the problem persists, then restart normally.  Use safe mode to isolate issues with your Mac and Playing Safe - what does Safe mode do?
7. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
8. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
9. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
10. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Install OS X then click on the Continue button.
11. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Install OS X and click on the Continue button.

It did not work even after you erased the drive and installed High Sierra? Are you sure the drive is OK? Perhaps it would be a good idea to run the Apple Hardware Test: Apple Hardware Test.

You could also make a backup of your files then do the following:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

This should install Catalina, but it may only install High Sierra.

Quit the operation. See How to upgrade to macOS Catalina.

I suggest your also run First Aid before downloading and installing Catalina. First Aid- Verify and Repair HFS+, APFS Drives with Disk Utility.

Kappy I have opened disk utility up and ran first aid on Apple SSD, Container disk1 and Macintosh HD as that was my only 3 options.

I have then tried to download it again and it has not worked.

I then tried staring up in safe moss and Downloading and that has not worked any more advice ?

I didn’t erase the whole drive, but I found a “high Sierra install” file that was corrupt. Deleted it. Now actually installing the correct “High Sierra” right now. I’m hoping that will then enable me to download “Catalina”, or at least download something further up the ladder. Thank you for your help!

DUUDE! SOMETHING FINALLY WORKED. I went to System Preferences -> Security and Privacy -> FileVault -> Turn Off FileVault

This will decrypt everything on your computer (took about 12 hours for me). I don’t know the effects it might have, but it frickin worked to download the security update 10.13.6. IT STILL WONT WORK FROM THE COMBO DOWNLOAD. The same error message will show. So I put the combo install into the trash, and I just went to the App Store and it finally downloaded the Security Update 10.13.6.

I’m currently downloading Mojave and then after I have all that I need I will turn FileVault back on. I hope this helps!!

So, I turned my file vault off TO GET 10.13.6. Now it's still not letting me get Catalina, but I'm wondering if Apple is starting to address the issue now. Because now I go to About This Mac -> Software Update -> and then it shows that my software is up to date. ALSO, if I try and search Catalina in the App Store, hit get, it then takes me to the same "Software Update" page, only to show me an error message "The requested version of macOS is not available", before then showing me that my software is up to date. I really don't know where to go from here, but the apps I needed to work are working. I'm gonna take it as a victory. Best of luck to the rest of y'all!!!

JonSchaef, maybe now try to start in Safe Mode as Kappy described. Last night I had the exact same problem you describe, and Safe Mode did the trick.

Keep in mind that you might have to hold down the shift key for a really long time. I thought it had stalled and I was about to give up, but it eventually started.

The only issue (minor) I have now is that the red dot on System Preferences that says I have a software update, even though it shows that I have the latest Catalina 10.15.4 now. I'm searching for help on this one now...

Mine has worked mate thanks to Kappy I turned it off then turned it on held alt-cmd-R then wait and connect to internet and then just clicked install OS think it’s the second one down. It didn’t work when I tried this yesterday but it has now worked thanks Kappy.