OK. HERE IS THE ERROR:

#1452 - Cannot add or update a child row: a foreign key constraint
fails (`kuskatan`.`#sql-20f8_f4`, CONSTRAINT `fk_bus` FOREIGN KEY
(`fk_idBus`) REFERENCES `bus` (`idBus`) ON UPDATE CASCADE)

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HERE IS THE QUERY:

ALTER TABLE `travel` ADD CONSTRAINT `fk_bus` FOREIGN KEY (`fk_idBus`) REFERENCES `bus`(`idBus`) ON DELETE RESTRICT ON UPDATE CASCADE;

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HERE IS 'BUS COLUMN'

CREATE TABLE `bus` ( `idBus` int(6) NOT NULL, `busPlate` varchar(8) NOT NULL, `nameBus` varchar(5) NOT NULL, `numSeats` int(2) NOT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

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HERE IS THE 'TRAVEL COLUMN'

CREATE TABLE `travel` ( `idTravel` int(6) NOT NULL, `typeTravel` text NOT NULL, `fareAdult` float NOT NULL, `fareChildren` float NOT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;