I don’t think I can see an error in your screenshot? Only the confirmation box.
If everything seems as you want it to be (for ubuntumate to be displayed as the tag on the ubuntu-mate tagged topics instead) then press OK to make it so.
Perhaps I have misunderstood what tag synonyms do.
I no longer want ubuntumate to be used as a tag but instead if someone selects it then it should redirect to ubuntu-mate, which will then be the primary tag.
Or should I just rename ubuntmate and all its topics will then be updated anyway?
I was looking for a way not to delete or rename it so that we have some kind of archival resource but simply redirect the tag in future.
This sounds like you want to create the synonym on the other tag’s page instead, as Putty said.
If you try that (when you’re ready ) and see if the information in the confirmation box is more what you had in mind.
Though this could be another one where you could create a couple of dummy tags on some test topics in a private category and play with the synonym feature with those to get a better feel for what the result will be.