Problem with trigger

[Ismael Alexander Malca Castro]

Hi!, I need execute a trigger when only 1 field of my table alumnos be updated,
the field name is 'deuda' (without '' obviously) I need to execute the trigger when this field be updated, this is my code

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DELIMITER $$
CREATE TRIGGER Tpago
BEFORE UPDATE on alumnos.deuda
FOR EACH ROW
BEGIN
UPDATE `alumnos` set `deuda` = OLD.deuda - NEW.deuda
where `alumnos`.`a` = OLD.a;
END $$
DELIMITER ;

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And MySQL error is
#1146 - Table 'alumnos.deuda' does not exist!.

I really apreciate ur answers thx!