Check if your PHP files are created at the correct location.
Namespace:
Illegal namespace!
Class name:
Illegal class name!
Fully-qualified class name: Illegal fully-qualified class name!
Relative file path in plugin project: Not a valid class file!
So, you should have a file at with this content:
<?php
namespace ;
// use statements...
class /* extends ParentClass */{
// content...
}
And, if it is the main class, you should have this line in plugin.yml:
main: \