That is actually a known bug on AO3's part that we warned people about at the top of this post. What you can do is copy/paste the exact tag from the tagset, hit enter, and AO3 should accept it. It has to be an exact match though - extra spaces or punctuation marks will cause a rejection.
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That is actually a known bug on AO3's part that we warned people about at the top of this post. What you can do is copy/paste the exact tag from the tagset, hit enter, and AO3 should accept it. It has to be an exact match though - extra spaces or punctuation marks will cause a rejection.