My Mac mini won’t update

Use Safari if you were using a third party browser to download the update.

If that doesn’t help, startup in Safe Mode and try again.

Lastly if your running any kind of third party AVware or so called Cleanware, those could be messing up your macOS’s ability to update properly.

Download and run the 10.15.7 Combo Updater directly from Apple Downloads.

https://support.apple.com/kb/DL2052?viewlocale=en_US&locale=en_US