It sounds like you might be trying to use the prompt form instead of the nominations form for the tagset. You can only use the prompt form to request things that are already in the tagset, but you can still add new nominations to the tagset and then request those once they're approved. It'll just take a little time for them to go through.

We also don't accept any / relationship tags, for the record: please nominate any relationships as & relationships and then specify in your prompt if you're open to shipfic for that tag.