While re-creating a spatial index it failed as follow:

DROP INDEX TEST.SIX_POLYGON;

CREATE INDEX TEST.SIX_POLYGON ON TEST.POLYGON (GEOLOC) INDEXTYPE IS MDSYS.SPATIAL_INDEX PARAMETERS('TABLESPACE=INDEX_TS') NOPARALLEL;

ORA-29855: error occurred in the execution of ODCIINDEXCREATE routine
ORA-13249: internal error in Spatial index: [mdidxrbd]
ORA-13249: Error in Spatial index: index build failed
ORA-13249: Error in R-tree: [mdrcrtscrt]
ORA-13231: failed to create index table [MDRS_1EEB04$] during R-tree creation
ORA-13249: Stmt-Execute Failure: CREATE SEQUENCE "TEST".MDRS_1EEB04$ ORDER START WITH 1 CACHE 100
ORA-29400: data cartridge error
ORA-01031: insufficient privileges
ORA-06512: at "MDSYS.SDO_INDEX_METHOD_10I", line 10

After some digging in, I found the missing privilege: create sequence. You can check if the user has required rights by:

select privilege
from DBA_SYS_PRIVS
where privilege in ('CREATE TABLE', 'CREATE SEQUENCE' )
and grantee = 'TEST';

All needed is to grant create sequence to the target user before creating index. Afterwards you can revoke the right:

GRANT CREATE SEQUENCE TO TEST;

CREATE INDEX TEST.SIX_POLYGON ON TEST.POLYGON (GEOLOC) INDEXTYPE IS MDSYS.SPATIAL_INDEX PARAMETERS('TABLESPACE=INDEX_TS') NOPARALLEL;