Error Moving Tag into a folder

I currently have a folder in the tag browser with a lot of tags in it.

I recently added a new tag that I want to put within the folder but every time I try, I get this error:

Even though the folder already has tags in it. Is it because the folder is apart of a UDT?

Any idea what could be causing this?

Yes, thats not a folder, but a tag instance of a UDT type. If you need to add an additional element, modify the UDT.

Ok, just needed to confirm that was the issue. Thanks for the help.