I am running macOS Mojave version 10.14.6 on a 27 inch iMac, late 2013; I want to update to macOS Catalina version 10.15.7. When I use "Upgrade Now" the system tries to update, but then it does not change.
iMac 27″, macOS 10.14
First have a good backup.
Start up from macOS Recovery
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
In Disk Utility>View, select Show all Devices, highlight the top left entry.
Run Disk First Aid on all items in the left panel, from top down.
Then try the install.
System will not update
