Public Function GetCertificado() As X509Certificate2
    Dim store As X509Store = New X509Store
    store.Open((OpenFlags.ReadOnly Or OpenFlags.OpenExistingOnly))
    Dim collection As X509Certificate2Collection = CType(store.Certificates, X509Certificate2Collection)
    Dim fcollection As X509Certificate2Collection = CType(collection.Find(X509FindType.FindByTimeValid, DateTime.Now, False), X509Certificate2Collection)
    Dim scollection As X509Certificate2Collection = X509Certificate2UI.SelectFromCollection(fcollection, "Certificados", "Seleccione un certificado", X509SelectionFlag.SingleSelection)
    If (scollection.Count = 0) Then
        Return Nothing
    End If
    Return scollection(0)
End Function

Public Sub Sign(cert As X509Certificate2, route as String)
    Dim doc As XmlDocument = New XmlDocument()
    doc.LoadXml(File.ReadAllText(route))

    Dim signedxml As SignedXml = New SignedXml(doc)
    Dim reference As Reference = New Reference()
    reference.Uri = ""
    Dim env As XmlDsigEnvelopedSignatureTransform = New XmlDsigEnvelopedSignatureTransform()
    reference.AddTransform(env)
    signedxml.AddReference(reference)

    Dim KeyInfo As KeyInfo = New KeyInfo()
    KeyInfo.AddClause(New KeyInfoX509Data(cert))
    signedxml.KeyInfo = KeyInfo
    signedxml.SigningKeyName = cert.Subject

    signedxml.SigningKey = cert.PrivateKey '<--- origen de error'
    signedxml.ComputeSignature()
    Dim xmlsig As XmlElement = signedxml.GetXml()
    doc.DocumentElement.AppendChild(doc.ImportNode(xmlsig, True))
End Sub