ORA-01950: no privileges on tablespace 'USERS'

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.

found the password for the table owner. Tried creating constraint as the owner of the table. Still getting the same error. Granted unlimited quota on users. Still getting error.

log-in as system,
alter user pcx quota unlimited on users;