I cannot create a lobby on the custom region. When I click "confirm", either nothing happens or the screen just turns black and it disconnects me.

You have to configure a custom server first.
Open BepInEx/config/org.bepinex.plugins.SheriffMod.txt
and change the IP address to the IP address of a custom Server. Atm, i do not host a specific custom server for this mod.
However, you can easily host your own server with https://github.com/Impostor/Impostor