CREATE OR REPLACE TRIGGER check_join_date
        BEFORE UPDATE OF join_date ON DOCTOR
        FOR EACH ROW
        WHEN (NEW.join_date > (SELECT CURRENT_DATE+1 FROM DUAL) )
    BEGIN
        RAISE_APPLICATION_ERROR(-20509,'Do not Enter Future Date..');
    END check_join_date;
    /

ORA-02251: subquery not allowed here