Since this bug is still unfixed on the archive, I'm repeating
elynross's post from last year.
Some characters that are available in the tag set are not appearing as characters to choose in the character autocomplete.
These characters are still available for requests/offers, but you'll have to enter them manually. The best way to do this is to copy and paste them exactly as they appear in the tag set, qualifiers and all. If they don't match exactly, then they won't be recognized by the matching code, and if they have any spelling or punctuation errors, your signup may not go through.
Before offering to write "Any" characters in a fandom, make sure you've checked the fandom listing in the tag set, not just in the autocomplete. You will be responsible for all nominated characters in that fandom!