How do you remove a tag from a post?

So simple question - have a blog post - with 5 tags. I'd like to remove one as it doesn't apply to the context of the post.

Whenever I click on the 'X' to remove the tag, it just comes up with a white box. It never deletes the tag.

I assume it wants to have 5 tags (my default), so I put in another relevant word, now I have six tags. I can't delete the tag I intended.

What am I doing wrong?