There have been instances when the SharePoint super users behaves adventurous and test SharePoint capabilities. As a result of their playing around and testing the system, they create entries here and there and then forget about them completely. One such user came to me with an error “Something went wrong” (default SharePoint error message) while he was trying to add content type to the document library.
Upon some debugging and looking into the logs (fortunately it was On-prem) it came out that the sub-site had duplicate columns which were created by the very same user who came to me with the issue. It was added to all sorts of different content types and referenced in couple of places. I had to clean all that before being able to delete that column.
Once I got rid of the duplicate column, I was able to add content type to the library again happily.
Hope this helps someone as with cloud solutions it can prove difficult with no availability of log files.