It should be:-
insert into suppliers (supoliers_no,supply_name,city,phone_no) values (1,'Ali','amman',111111);
insert into suppliers (supoliers_no,supply_name,city,phone_no) values (2,'tariq','amman',777777);
insert into suppliers (supoliers_no,supply_name,city,phone_no) values (3,'mohmmed','taiz',null);
Also insert into suppliers (supoliers_no,supply_name,city,phone_no) values (3,'mohmmed','taiz',null);
as null
is not a recognized keyword in sql. So you might have to try it out like insert into suppliers (supoliers_no,supply_name,city,phone_no) values (3,'mohmmed','taiz','');