In code bellow I'm trying to create Bitcoin payment:

var privateKey = "f267c05d4552b92825e46cfe056d6e3ff36072538f71d8c5acf3ebc4b09f4c87";       var 
key = new Key(Encoders.Hex.DecodeData(privateKey));
Uri BitPayUri = new Uri("https://test.bitpay.com/");
static Network Network = Network.TestNet;
BitcoinSecret k = new BitcoinSecret(key, Network);
Bitpay BitPay = new Bitpay(k.PrivateKey, BitPayUri);

var invoice = BitPay.CreateInvoice(new NBitpayClient.Invoice()
{
            Price = 50,
            Currency = "USD",
            PosData = "posData",
            OrderId = "123",
            RedirectURL = "redirectURL",
            NotificationURL = "NotificationURL",
            ItemDesc = "description",
            FullNotifications = true
});
return Redirect(invoice.Url);

When I try to call BitPay.CreateInvoice(...), I get an error:

NBitpayClient.BitPayException: 'Error: System.MissingMethodException: Method 
not found: 'NBitcoin.Crypto.ECDSASignature 
NBitcoin.Key.Sign(NBitcoin.uint256)'.

at NBitpayClient.Extensions.BitIdExtensions.GetBitIDSignature(Key key, String uri, String body)

Error says my key is not signed yet if I'm correct, but dont know how to fix it.

P.S. Code example would be useful.