Hello Sanket,
please can you add an exception handling to your method call to find out more details (add exception class CX_AMDP_ERROR to the declaration fo method GET_MET_DET2 and catch the exception when the method is called; check with methods GET_TEXT, GET_LONGTEXT if more details are provided).
Another point is how you work with the client. It is not correct that the client is an exporting parameter in the AMDP method. You have to declare it as importing parameter to be able to use it successfully with the correct client value in your where clause. As you have used already the naming convention IM_CLIENT I think it was already in your mind to do so.
Regards,
Florian