Hi joycefromwoodbury,
Before doing an upgrade make sure you have a good know backup of your data.
Try booting into Safe Mode and see if you can do the upgrade from there.
Hold down the Shift key when you boot up your Mac. You can also refer to the article, How to boot into Safe Mode if needed.
Safe boot take a little longer to boot as performs a disk repair and certain system caches get cleared and rebuilt, etc.
Another thing you can try is booting into Internet Recovery by holding down: Option Command R on your keyboard until you see a spinning globe. This will install MacOS Catalina over Mojave, retaining all of your data and applications.
You can also refer to the following article if needed: How to reinstall macOS from macOS Recovery.
I hope this helps.