this happened to me but i had module manager so i deleted the mod it said was not compatible and it worked. just for anyone that has this problem and stumbles across this thread
I got real solar system and its dependencies off of ckan, but when i loaded ksp up (1.8.1) kopernicus gave me a warning and said it wont work. It might have been because I had realism overhaul and a bunch of other mods installed, so I'll try again.