Hi Kiran,
If you observe your read statement, you are comparing the field with it's own table field.
Read table i_bvtyp with key bankn = i_bvtyp-bankn .
Untill unless you read I_BVTYP, i_bvtyp-bankn will be initial and you cannot find the matching record with Null-Value.
So, I think you need compare with l_bankn.
Read table i_bvtyp with key bankn = i_bankn .
Then you can directly raise a message using SY-SUBRC value.
Read table i_bvtyp with key bankn = i_bankn
if SY-SUBRC <> 0 .
<MESSAGE>
endif.
Regards,
Vijay