I changed the field to a note and it all worked. It will not fire on constraint on the field that is just a calculate since it does not show on the screen.
As you see above though I do not think this is what you want as this requires each task to be 8. You need a total outside the repeat and a constraint on that. And you need to bind to decimal otherwise 123 thinks its a string. (or use decimal then hidden appearance) See attached.
But in the end why are you making one hours field for each type? This form could be way, way simpler just having a hours field. See second attached. You repeat table will show which task it was. Going forward your original schema is going to be a nightmare to use. Basically you have to check all those fields all the time. Just use one hours field is the better schema way to do it.
Hope that helps