Hello,
I'm quite stuck. I know there doesn't seem a way to get the current item in a ForAll loop so that we have to resort to LookUp.
I already found this answer.
https://powerusers.microsoft.com/t5/Building-Power-Apps/Current-record-inside-the-ForAll-function/td-p/412988
The problem is that I need the item from the collection I'm looping over. This code doesn't work:
ForAll(colTeams,
ForAll(TimeSlots,
If(IsBlank(Tijdslot), Patch(Tijdsloten, Defaults(Tijdsloten), LookUp(TimeSlots, 'Zone ID' = TimeSlots[@'Zone ID'])))
)
)
It gives me this error:

This code on the other hand works perfectly:
ForAll(colTeams,
ForAll(TimeSlots,
If(IsBlank(Tijdslot), Patch(Tijdsloten, Defaults(Tijdsloten), LookUp(TimeSlots, 'Zone ID' = 3)))
)
)
Please tell me I don't have to rename the column like in this answer:
https://powerusers.microsoft.com/t5/Building-Power-Apps/Reference-Current-Record-of-ForAll/td-p/162054