Unable to view partition table in a schema
If you created the table with the above statement as user MDR_139, then that table belongs to the MDR_139 schema, not TESTSCHEMA.

If you want to create tables in other schemas, add the schema specifier to the create statement, for example:

create table TESTSCHEMA.sales_test (year number(4),
                    product varchar2(10),
                   amt number(10,2))
     partition by range (year)
     (partition p1 values less than (1992),
     partition p2 values less than (1993),
     partition p3 values less than (1994),
     partition p4 values less than (1995),
   partition p5 values less than (MAXVALUE));