basically what this is saying is that there is another asset or c++ class in that project that needs to be imported with that asset or else it wont work. Also, ive found that trying to import while the game is open also usually doesnt work. what you should do is close your project, then go to the root folder (its in documents/unreal projects unless you changed it) then go into the content folder and drag them in there from the other project manually. If your not sure which asset it needs just import all of them. This should fix your problem.