Unable to add existing license in non-standard location to project

Please note that this question is about project licenses, not GitLab activation licenses.

My problem seems to involve multiple issues.

First, some of my project pages don’t even have an “Add license” button, nor is there a license displayed.

Second, selecting “Add license” brings me to a page to add a new license. I am not able to select an existing file containing the license information.

I have searched the project preferences and not found anything (perhaps I missed it). I have also done multiple web searches, but most results yield GitLab activation licenses help (hence the first line of this topic).