Error in Excel/VB function: “Constant expression required”

You can’t DIM against a variable size. ReDim it instead

For example

Function binomial(n As Integer, p As Double)
Dim probabilities() As Double
ReDim probabilities(0 To n)
MsgBox LBound(probabilities)
MsgBox UBound(probabilities)
End Function

Sub test()
Call binomial(3, 2)
End Sub

Run the sub “test”