It simply means two things:
- The Editor is unable to load your compiled project, for which assets in the Editor depend on.
- You are not able to compile your project.
There is some problem with your code that is preventing compilation. Solving these problems and successfully compiling your code will fix the issue.
You should be able to right-click your .uproject file and Generate Visual Studio project files, this will give you a Visual Studio solution that you can open. Then attempt to compile your project, and resolve any compilation issues that come up.