"compile error" show sheet based on entering password by input box

Kalil

InputBox returns a string or a null string but your (incorrect and uncompiled) IF test is expecting a TRUE return (if we forget whatever undefined LT&GT are meant to be).

Try replacing your code with the code below, noting I've declared variables at the workbook level. It will keep the user on the STOCK sheet until the password entered. Key changes are in bold: 

Option Explicit
Dim PvSh As String, Pwd As String

Private Sub Workbook_SheetActivate(ByVal Sh As Object)

Dim Num As Integer

If Pwd = "" Then
    If Sh.Name <> "STOCK" Then
        Num = ActiveWindow.Index
        Windows(Num).Visible = False
        If InputBox("Enter Password", "Password", "????") <> "airplane" Then
            MsgBox "Incorrect Password", vbCritical, "Error"
            Application.EnableEvents = False
                Windows(Num).Visible = True
                Sheets("STOCK").Select
            Application.EnableEvents = True

            Else
            Pwd = "airplane"
        End If
        Windows(Num).Visible = True
    End If
End If
End Sub


Private Sub Workbook_SheetDeactivate(ByVal Sh As Object)

PvSh = Sh.Name

End Sub

Hope this helps.