Need Help with DAX IF Condition

Ah. Yeah, measures shouldn't need extra aggregations.

I think your measure could be simplified quite a bit. I'd suggest starting from something like this:

MeasureName =
VAR Channel = SELECTEDVALUE ( Input[Channel] )
VAR Customer = SELECTEDVALUE ( Input[Customer] )
VAR Brand = SELECTEDVALUE ( Input[Brand] )
RETURN
    IF (
        Channel = "All"
            || [NR/HL Country Base] = 0
            || CONTAINSSTRING ( Channel, "CENTRAL" )
            || CONTAINSSTRING ( Customer, "CENTRAL" )
            || Brand = "RESIDUAL STOCK"
            || CONTAINSSTRING ( Brand, "CENTRAL" )
            || ABS ( [NR/HL Channel Base] - [NR/HL Channel AC] ) > 500,
        0,
        1
    ) / 1000

Note that CONTAINSSTRING is not case-sensitive, so you don't need upper.