There're some restrictions on using index-organized tables (IOT). This is one case of them.
SQL> alter table ERPAPP.PAY_EXT_P drop constraint XM500P0;
alter table ERPAPP.PAY_EXT_P drop constraint XM500P0
ERROR at line 1:
ORA-25188: cannot drop/disable/defer the primary key constraint for index-organized tables or sorted hash cluster
ORA-25188 error message is pretty straight forward, we can't drop, disable defer any IOT's primary key constraint, because it's a required component of an index-organized table.
How to avoid ORA-25188?
You can query ALL_INDEXES in advance to identify IOT.
SQL> select * from all_indexes where index_type = 'IOT - TOP';
The result is a list of IOT, the type is IOT - TOP.