Added mods friends cant join server

started a server and can load in and play base game with friends. when i add a mod it loads me in and i can use it. When my friend tries to join it just says START on his sceen. looking at the log it shows him connect and be disconnected from the server. If i delete mod we can join fine. Tried mods from modland and beam page and nothing works.

Where are you placing the mod files?

I go into the server folder, in to resources, and add mods to client folder.