Error: Module not specified (IntelliJ IDEA)

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:

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:)

For IntelliJ IDEA 2019.3.4 (Ultimate Edition), the following worked for me:

1. Find the Environment variables for your project.
2. Specify, from the dropdowns, the values of “Use class path of module:” and “JRE” as in the attached screenshot.

I faced the exact issue of module not found and solved it by navigating to pom.xml file in file system of my project and opening it as a maven project in my intelliJIdea.

Then it has populated the sub modules for selection in the edit configurations window.