macOS Catalina 10.15.6 update has restart loop, machine does not get updated, is there a reason why?

That is a supplemental update to the 10.15.6...

did you install the supplemental?

The stand alone

Supplemental update 10.15.6 https://support.apple.com/kb/DL2049?locale=en_US

(non Safari browser have been having an issue w/ Apple system downloads ex Firefox—therefore try Safari. )

You can proceed with getting past the loop, by booting into SafeBoot mode and try again your update

SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Login and update.

reboot as normal

Thank you! I made sure I had room (yes), downloaded the standalone 10.15.6 Supplemental Update (your link / Apple site) and when it finished at least I saw a message saying some updates couldn't be installed manually. So I SafeBooted and did the update, then when I finally rebooted again in normal mode it really was updated.

wer2020 wrote:

Thank you! I made sure I had room (yes), downloaded the standalone 10.15.6 Supplemental Update (your link / Apple site) and when it finished at least I saw a message saying some updates couldn't be installed manually. So I SafeBooted and did the update, then when I finally rebooted again in normal mode it really was updated.

Good report wer2020 , sounds like this solved your problem.