My shop won't let me add another category. I only have 4 and it says your allowed to have 20. Does anyone know what I should do?

The categories have recently gotten an update, making it possible to rename your categories. It has been glitched since, there were a couple of other topics about it the last few days.

so should I just wait and try later or what?

Yes, that would be best, there are a couple of topics about this in Problems & Bugs so I assume the staff is aware of the problem and that they are going to fix this soon =)

ok thank you :]