I cant host without picking a map, but when i do name the map it still doesn't work, what am i doing wrong?

In vanilla/standard jj2 hosting co-op isn't really possible and has many bugs.

I strongly suggest using the community add-on patch called JJ2+ that you can find in your GOG settings. Here are the steps to do so. https://gog.jj2.plus/

In JJ2+ you can also type in game with 'T' to type in several server commands, which you can find here: https://docs.jj2.plus/#commands

Hope this helps!