If statement Compile error

I'll try to make a macro with an If statement in it. But I get the error message

Sub or Function not defined

The following code is used.

Sub whatif()

    If Range("memory") = "data" Then 
Worksheet("Side").Range("B1") = "yes"
    End If

End Sub

I figured out the problem is line

Worksheet("Side").Range("B1") = "yes"

When commenting out the other lines it still gives an error. What does need adjustment?