Consuming SOAP webservice showing <error>invalid credentials or invalid request format</error>

See more:

XML

SOAP

webservice

C#7.0

I am consuming SOAP web service from my .net application. I have executed the .wsdl file given to me and it is showing the definition content. I have following code that is not giving required response.

static void Main(string[] args)
        {
            TestService();
            Console.ReadLine();
        }

        private static async void TestService()
        {
            try
            {
                ServiceReference1.SolutionsClient client = new ServiceReference1.SolutionsClient();
                client.ClientCredentials.UserName.UserName = "username";
                client.ClientCredentials.UserName.Password = "password12345";

                var result = await client.SolutionAvailabilityAsync("", "", "", "", "", "2019-05-15", "2019-05-16", "", "", "2", "1");

                string value = "a";
            }
            catch (Exception ex)
            {

            }
        }

In result, I am getting <Error>Invalid Credentials or Invalid Request Format</Error> as @return. It expects every parameter in string. What is the cause of this error???

What I have tried:

Tried changing the date time format but it did not help.