Do you mean that you don't get an autocomplete for it as you start typing? That's a known AO3 bug that's been around for years now. If you type the tag in full, or (probably easier) cut and paste it from the tag set/spreadsheet, it should still work even though it looks like it doesn't know it's there.

If it's not that, and instead it's that even after trying that you can't successfully save your sign up, then that's a more serious issue - if you encounter that, sign up without her and let me know; I'll see if I can edit your sign up after you've created it to add Black Widow in. But to be honest I'm 95% sure it's the first thing (especially as I've just had a quick try with my account and she doesn't show up as a drop down for me either).