Run configurations not working

Whenever I open a .java file and try to create a run configuration, I get this error: "Warning: Class '(my class)' not found in module '(my module)'". I am putting in the class that contains my main method (for example, "public class Dog" I would enter "Dog"), and the problem is on my other computer, too. I had solved it by just making a new project and copying the code from the .java file into it, but today when I reopened a project that was working the other day, I had the same problem. Any help would be appreciated.

Thanks