Error occurred during initialization of boot layer

I keep getting this error:

Error occurred during initialization of boot layer
java.lang.LayerInstantiationException: Class loader (instance of): 'app' tried to define prohibited package name: java

This happens when I tried to change the structure of my files, I separate then into folders but then error occurred. I decided to bring it back, but the error keeps getting in the way. The program is not working now. I hope someone could help me. Thank you so much :)