Non-System Font Identified as System Font

What system were you running when you got the free version of the font? If it was Mojave or earlier the fonts were stored in a volume which became read only in Catalina and newer. That's why you can't remove the font. Can you add the purchased font to Font Book and then add it to the Favorites portion of Font Book. That will put it at the top of the font list in many app's font menu, making it easy to find and distinguish from the free version.

Coyote--Coyote wrote:

A long time ago I downloaded the free version of a font, I decided to buy a license for it and now cannot replace the free version with the licensed version.

Font Book is reporting the free version of the font as a System Font so I can't remove it or overwrite it with the licensed version. Unfortunately, I can't find the free font anywhere on my system so I can't delete it manually. I've searched for the font name and can only find the newly downloaded version, so I manually searched the various font folders (/Library/Fonts, /System/Library/Fonts, and /Users/user/Library/Fonts) and didn't find it.

When I use Font Book to remove the licensed version of the font both the licensed version and the free version disappear from the font list. But when I try to re-install the licensed version the free version shows back up as a locked System Font.

ref: Install or remove fonts on your Mac - Apple Support