You will need to fill in the address into the string.
.Formula = "=COUNTIF(" & ReqItems.Address & ","">0"")"
Please also note that
Dim ReqItems, SupItems As Range
only declares SupItems As Range
but ReqItems
will be of type Variant
here. You will need to declare a type for every variable.
Dim ReqItems As Range, SupItems As Range
0
solved Excel VBA – Referencing a named range in a formula