ORA-08104: this index object string is being online built or rebuilt

Run the below script with the object id and your problem will be resolved.

DECLARE
RetVal BOOLEAN;
OBJECT_ID BINARY_INTEGER;
WAIT_FOR_LOCK BINARY_INTEGER;

BEGIN
OBJECT_ID := 63556;
WAIT_FOR_LOCK := NULL;

RetVal := SYS.DBMS_REPAIR.ONLINE_INDEX_CLEAN (OBJECT_ID);
COMMIT;
END;

After running the above script your index will be ready for use.