How to use Foreign key (Problem)

today I am facing this problem for Foreign key,
ALTER TABLE products add FOREIGN key (suppliers_id) REFERENCES suppliers (Sup_id)

`#1452 - Cannot add or update a child row: a foreign key constraint fails (`codewithcpn`.`#sql-1bfc_46b`, CONSTRAINT `#sql-1bfc_46b_ibfk_1` FOREIGN KEY (`suppliers_id`) REFERENCES `suppliers` (`Sup_id`))`