subtracting 1 from the 'Created on' field is not delegable. You should do the manipulation to the variable in Power Apps rather than the field in your source.
Also, I assume you are subtracting 1 day? you can change it to this:
Sort(Filter(KPUnblocks, ('Created On' >= DateAdd(StartDate.SelectedDate), 1, Days) && ('Created On' <= DateAdd(EndDate.SelectedDate, 1, Days)) && (ItemFilter.Text in ItemNumber)),'Created On',Descending)
You also have the "In" operator, which is unfortunately non-delegable with Sharepoint(I assume that is what you are using).
The only way I see this being resolved is using a collection, so in App OnStart you can do
ClearCollect(MyCollection, KPUnblocks)
and then use MyCollection for all your needs in the Power App.