Once you have enough reputation, you can create new tags for questions that don't have enough tags. Levels vary, but you generally need 150 rep on a beta site or 300 or 1,000 rep on a graduated site to create new tags on that site - levels vary.
In the meantime, you have three options:
- Wait to ask the question until you have enough rep to create new tags. Unfortunately, this is not possible on mobile, no matter what rep you have.
- Choose a tag that you think is related enough to the question (do some digging!) and leave a note at the bottom of your question asking for tag suggestions. You could also do this in a comment, if you wanted. I've done this before, even on sites where I had enough rep to create new tags.
- Ask people in chat. There, you can get advice on what tags to use, or - if that is unsuccessful - perhaps a user will volunteer to create a new tag for you.
The third option may be the most successful, but only use it if you're positive that there are no tags that suit the question. Definitely do some digging first.
It looks like the same advice has been given before. . .