I am getting an error while adding constraint on a table.

I don't know the password for the table owner. So logged in as SYSTEM and ran the command below.

ALTER TABLE pcx.PCX_PC_TRANS_HISTORY ADD(
CONSTRAINT PK_PCX_PC_TRANS_HISTORY PRIMARY KEY (AS_OF_DATE, OFFICE_CODE, PMS_CODE, TRADE_TYPE, CHNL_CD));

Getting ORA-1950 error.

ALTER TABLE pcx.PCX_PC_TRANS_HISTORY ADD(
*
ERROR at line 1:
ORA-01950: no privileges on tablespace 'USERS'

I granted unlimited quota on USERS. Still getting the error.

Any ideas? Thanks.