Error: Module not specified (IntelliJ IDEA)

this is how I fix this issue
1.open my project structure

2.click module 3.click plus button 4.click import module，and find the module's pom
5.make sure you select the module you want to import,then next next finish:)

This is because the className value which you are passing as argument for
forName(String className) method is not found or doesn't exists, or you a re passing the wrong value as the class name. Here is also a link which could help you.

1. https://docs.oracle.com/javase/7/docs/api/java/lang/ClassNotFoundException.html
2. https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#forName(java.lang.String)

Update

Module not specified

According to the snapshot you have provided this problem is because you have not determined the app module of your project, so I suggest you to choose the app module from configuration. For example: