User Defined Function in Excel

I have attempted to put in a UDF in three different machines and get the same result. I get an error when I attempt to reference the UDF. I put in
Function AddUp(x,y)
AddUp = x + y
End Function

When I pick a cell in the worksheet and put in
=AddUp(5,2) I get an error #Name? I have unchecked the Require Variable Definition box so why won’t it run?