I purchased this version of Primis, and it says it is compatible with 1.18.1+.
My server supports versions from 1.18+ to 1.20.1, but when I try to install Primis, I get an error related to version incompatibility.
How can I contact the seller to request assistance or a refund?

On the page where you purchased the product, at the bottom of the page, you'll find an area for comments and questions. If not, the author should appear under "More Information." If you click on it, you should see their profile, where contact information is usually provided.

If you can't contact them, you'll need to contact Itch support.