Error is occurring while provisioning customer domain for SharePoint online for Nintex workflow

We are migrating Nintex workflow from SharePoint On-Premise to SharePoint online.

During import,we are provisioning customer domain. At this time, we are geting an ArgumentException i.e. "One or More error" has occurred (we are not sure about inner exception for this). So do you have any inputs when this type of error i.e. "One or More error" can occur while provisioning customer domain? Below is the sample code that we have written to provision customer domain.

try
{
         HttpResponseMessage response = client.GetAsync(ProvisionEndpoint).Result;
         string json = response.Content.ReadAsStringAsync().Result;

         if (!response.IsSuccessStatusCode)
         {                     
                  response = client.PostAsync(ProvisionEndpoint, new StringContent(string.Empty)).Result;
                  response.EnsureSuccessStatusCode();
         }

         ProvisionResponse result = JsonConvert.DeserializeObject<ProvisionResponse>(json);
         string result = result.Data.Domain;
}
catch (Exception ex)
{

//Throwing exception ArgumentException that One or More error has occurred.
}