Attempt 1: Make sure that you have opened up the correct java file that contains the
main(String args) method before hitting the green launch button.
Attempt 2: Right click on the project, and do Build Path -> Configure Build Path. Is it missing anything (like the opencv user library?) or wrong Java version (1.8 vs 10?)
Attempt 3: Try doing a Project->Rebuild or Clean. Try looking at Window->Show View->Problems, to see if it gives more information about what's wrong.
Attempt 4: I believe this sometimes happens when you imported the project incorrectly into eclipse.
Try removing the project from Eclipse, and importing again, using this proper method is:
1) Switch to the Git perspective in Eclipse to "add an existing local repository", so that Eclipse knows about EgretRepo.
2) Back in the Java perspective, do "Import" -> "Projects from Git", and browse to your EgretRepo, hit "next" a few times until it finds EgretProject.
Attempt 5 I think this can also happen if you opened the .java file directly from your hard drive, instead of opening it by finding it inside Eclipse and clicking on it there to open it.