You cannot explicitly drop a partition of a local index. Instead, local index partitions are dropped only when you drop a partition from the underlying table.
If a global index partition is empty, then you can explicitly drop it by issuing the ALTER INDEX DROP PARTITION statement. But, if a global index partition contains data, then dropping the partition causes the next highest partition to be marked UNUSABLE. For example, you would like to drop the index partition P1, and P2 is the next highest partition. You must issue the following statements:
ALTER INDEX npr DROP PARTITION P1; ALTER INDEX npr REBUILD PARTITION P2;