error while creating functions in pl/sql

create or replace
FUNCTION "XYZ" (P_ORDR_ID IN NUMBER)  
RETURN VARCHAR IS 
ab VARCHAR(15);
BEGIN 
SELECT tab3.ACNT_REF INTO ab
       FROM tab1,      tab2,
      tab3
    WHERE tab1.ORDR_ID                               = tab2.TXN_POS_NUM
    AND tab1.ORDR_POS_TYPE                           = tab2.TXN_POS_TYP
    AND TRIM(tab2.STLMNT_ACNT_POS_NUM)               = TO_CHAR(tab3.POS_ID)
    AND tab2.STLMNT_ACNT_POS_TYP                     = tab3.POS_TYPE
    AND tab1.BP_ID                                   = tab2.BP_ID
    AND tab1.STLMNT_CRNCY                            = tab3.POS_CRNCY 
    AND tab1.ORDR_ID                                 =  P_ORDR_ID;

RETURN (ab); 

     EXCEPTION
     WHEN OTHERS THEN RETURN (NULL);
END;