Look at the error message:
Overloaded method match for WhatsAppApi.Register.WhatsRegisterV2.RequestCode(out string, out string, string, out string)has some invalid arguments
Now look at how you are calling it:
if(WhatsAppApi.Register.WhatsRegisterV2.RequestCode(textBoxPhno.Text,out password,"sms"))
The method expects two
out
string parameters, a "normal" string parameter, then a final
out
string parameter.
You are passing it a string, an out string, and a string.
You need to match your parameters to the method signature. Consult the documentation for the method to work out what each parameter should contain!