Password Protecting Sheet

I've found the code shown to Password Protcet a sheet, but I keep getting a Compile Error saying Variable Not Defined, can anyone help please?

Option Explicit

Private Sub Worksheet_Activate()
Dim pwd As String:  pwd = "password"

Cells(Rows.Count, Columns.Count).Activate
    If Application.InputBox("What is the password to view this sheet?", "Access Password", "???") <> pwd Then
        Sheets("Sheet1").Activate
        MsgBox "Wrong password"
    Else
        Range("A1").Activate
    End If
   
End Sub