Why does macOS Catalina install and/or update fail with "OSStatus error -37"? Both after updating from macOS Mojave 10.14.6 to macOS Catalina 10.15.5 and again when updating from 10.15.5 to 10.15.6, the installation failed with OSStatus error -37. This failure leaves the computer in an endless update loop where it tries to perform the software installation, fails, reboots, and tries again. Forcing the computer to boot from the main internal drive fails: the computer is no longer bootable.
A clean install does work successfully. But, installing macOS Catalina (or a Catalina update) in-place fails.
How can I resolve the installation failure without wiping my computer clean and reinstalling from scratch?
I have a small internal drive in my 2018 Mac mini. So, I keep most of my data on a large external SSD. For ease-of-use, I replace most directories in the user home directories with symlinks. That way, for example, the ~/Documents dir points right over to a dir on the external SSD.
Well, I also use this symlink method for the /Users/Shared directory. It looks like:
&gt; cd /Users
&gt; ls -l | grep Shared
lrwxr-xr-x  1 admin    wheel  34 Jul 19 13:42 Shared -&gt; /Volumes/ExtSSD/shared

Restoring the /Users/Shared directory back to an actual directory — the way it looks as laid down by the macOS Installer — resolves the issue. Now /Users/Shared looks like:
&gt; cd /Users
&gt; ls -l | grep Shared
drwxrwxrwt  25 root      wheel  800 Apr 17 14:17 Shared

With /Users/Shared no longer a symlink, the macOS Catalina install/update process works fully and successfully.