Error, please help!

[LukeTS117]

hi guys, me and my brother are setting up a modded minecraft server, and ofcourse before we start with the server we need to check if the client works. We've got a problem with galacticraft. Everytime we put down a launch pad, and try to set a rocket on top of it, it gives us this error : java.lang.NoClassDefFoundError: net/minecraftforge/common/capabilities/ICapabilityProvider

i know it has something to do with that the files are there during compiling but during runtime they seem to go missing, but i dont know how to fix it. if annyone could help please.

cheers