What are you getting when you subtract the dates? A decimal number?
The result should be the number of days. To get it in days, minutes, etc... you will need to convert that yourself.
Here is an example for days/hours/minutes: Floor(([Fecha Apertura]-[Inicio de Afectacion])) & 'd ' & Floor(Fmod(([Fecha Apertura]-[Inicio de Afectacion]), 1) * 24) & 'h ' & Round(Fmod(Fmod(([Fecha Apertura]-[Inicio de Afectacion]), 1) * 24, 1) * 60) & 'm'
It should return in the format: 4d 3h 14m
Your dates are not dates or timestamps. They are text fields. You can't subtract text fields.
You can probably read them in as timestamps by changing your TimestampFormat to match:
SET TimestampFormat='DD/MM/YYYY hh:mm:ss';
But I can't test because you're loading from a file you didn't attach.
You can't put a load with no source after the main load and expect anything to happen. That sort of expression needs to be in the main load.
interval([Fecha Apertura]-[Inicio de Afectacion|Inicio de la interrupcion de servicio],'DD, hh:mm:ss') as [Tiempo de Apertura],
And that's assuming it interprets the timestamps correctly with the fixed format. Otherwise it gets more complicated.