Saw an error when we were trying to rebuild the index to another tablespace.
SQL> alter index sh.sales_cust_bix rebuild tablespace users online;
alter index sh.sales_cust_bix rebuild tablespace users online
ERROR at line 1:
ORA-14086: a partitioned index may not be rebuilt as a whole
ORA-14086 means that the index you want to rebuild is a partitioned index, you can't rebuild them as a whole since they're separate segments.
To solve the problem, you need to rebuild its partitions of the index one by one.