What got flagged?
Backup Link

For more context, "Backup Link" is the text on the hyperlink which is a backup(web archive) of the original link.

Why is this incorrect?
Backup in this case referes to the actual copy of the original link whereas "back up" is the phrasal verb for the action to make the copy which is not what's happening here.

harper mistakes the two in this case thinking a phrasal verb should be used:
This word should be a phrasal verb, not a compound noun.