How to create trigger to update timestamp when record is updated or inserted in another table
Thanks Michael for your help!

Good to know that @variables are users variables and without @ are local variables.

The following snipped worked well

USE `myTask`;
DELIMITER $$
CREATE DEFINER=`root`@`localhost` TRIGGER `t_taskaction_AUPD` 
AFTER UPDATE ON `t_taskaction` 
FOR EACH ROW
BEGIN
    DECLARE _idTask INT(11);
    DECLARE _idTaskState INT(11);

    set _idTask = NEW.idTask;

    set _idTask = (SELECT idTaskState 
    FROM t_task WHERE idTask = _idTask);

    UPDATE t_task 
    SET timestamp = now()
    WHERE idTask = _idTask;
END$$
DELIMITER ;