error ORA-14086: a partitioned index may not be rebuilt

error ORA-14086: a partitioned index may not be rebuilt as a whole occurs when you're trying to rebuild an index that is partitioned, but you're attempting to rebuild it as a whole (i.e., the entire index) rather than rebuilding it partition by partition.
In Oracle, partitioned indexes need to be rebuilt at the partition level rather than the entire index at once. If the index is partitioned and you attempt to rebuild it as a whole, you'll encounter this error.
To resolve this issue, you can rebuild the index partition by partition.
Steps:
Identify the Partitions:
SELECT INDEX_NAME, PARTITION_NAME
FROM USER_IND_PARTITIONS
WHERE INDEX_NAME = '<index_name>';
Rebuild Each Partition: Once you've identified the partitions,rebuild each partition individually using the following syntax:
ALTER INDEX <index_name> REBUILD PARTITION <partition_name>;
Replace <index_name> with your index name and <partition_name> with the partition name that you identified in the previous step.
Example:
ALTER INDEX my_index REBUILD PARTITION p1;
ALTER INDEX my_index REBUILD PARTITION p2;

Rebuild All Partitions in a Loop:  

If you have many partitions and want to rebuild them programmatically, you can loop through the partitions using PL/SQL. Here's an example script to rebuild all partitions:
    BEGIN
       FOR rec IN (SELECT PARTITION_NAME FROM USER_IND_PARTITIONS WHERE INDEX_NAME = '<index_name>') LOOP
          EXECUTE IMMEDIATE 'ALTER INDEX <index_name> REBUILD PARTITION ' || rec.PARTITION_NAME;
       END LOOP;
    END;
Alternative: Rebuild the Index with ALL Partitions (Oracle 12c+)
If you're using Oracle 12c or later, you can rebuild the entire partitioned index by rebuilding all its partitions in one command:
ALTER INDEX <index_name> REBUILD PARTITIONS ALL;
This command will rebuild all partitions of the partitioned index at once.
Example of Rebuilding a Partitioned Index
If you have a partitioned index emp_idx with partitions p2022, p2023, and p2024, you can rebuild them as follows:
Rebuild each partition:
ALTER INDEX emp_idx REBUILD PARTITION p2022;
ALTER INDEX emp_idx REBUILD PARTITION p2023;
ALTER INDEX emp_idx REBUILD PARTITION p2024;
Or, rebuild all partitions (Oracle 12c+):
ALTER INDEX emp_idx REBUILD PARTITIONS ALL;