ScriptManager As System.Web.UI.IScriptManager' is not accessible in this context because it is 'Friend'

Private Sub MyMessageBox(ByVal msg As String)
        ScriptManager.RegisterStartupScript(Page, Page.GetType(), "MyScript", "alert('" & msg & "');", True)
    End Sub

Getting error line under ScriptManager : showing Error mentioned below

Error24'System.Web.UI.Page.Friend ReadOnly Property ScriptManager As System.Web.UI.IScriptManager' is not accessible in this context because it is 'Friend'