'Value cannot be null. (Parameter 'input')' - CKECKOUT PAYMENT

I'm implementing checkout payment in C#, I have to get token from Checkout api, when I send public key I got error 'Value cannot be null. (Parameter 'input')' The code is below:

string publicKey = "pk_sbox_*******************";
            ICheckoutApi api = CheckoutSdk.Builder().StaticKeys()
    .PublicKey(publicKey)
    .Environment(Checkout.Environment.Sandbox)
    .HttpClientFactory(new DefaultHttpClientFactory())
    .Build();

What does cause this error? Any help will be welcome. With function StaticKey I don't have that error just in case when I use PublicKey