CHKJ2907E: Type com.test.SampleBean, or one of its supertypes, cannot be reflected. Check the classpath

I'm getting this error in my EJB project. I've searched for solutions and found the below solution everywhere

Right-click -> Properties -> Java Build Path -> Order and Export. Move ‘JRE System Library (jdk)’ to the top and have it check marked.

Right click the EJB project and click Validate.

For some projects it worked but for few it doesn't.

I don't understand what does the error exactly mean, I would like to know the root cause of it and know the proper solution.

Class or one of its supertypes, cannot be reflected.

Can someone help me understand what does this mean exactly and how do I try to resolve it?