Convert type 'string ' to 'system.net.http.httpresponsemessage'

public static async Task<string> GetByLastDate(string specificUrl, string token, DateTime date) // specificUrl like "user/" at the end "/"
{
var policy = Policy
  .Handle<Exception>()
  .OrResult<HttpResponseMessage>(r => !r.IsSuccessStatusCode)
  .RetryAsync(2);

   return await policy.ExecuteAsync(async() =>
   {
    string fullUrl = baseUrl + specificUrl + date;
      using (var requestMessage = new HttpRequestMessage(HttpMethod.Get, fullUrl))
       {
         requestMessage.Headers.Add("access_token", token);
         using (HttpResponseMessage res = await client.SendAsync(requestMessage))
          {
            string data = await res.Content.ReadAsStringAsync();
             if (string.IsNullOrEmpty(data)) { return string.Empty; }
               return data;
             }

         }
   }).ConfigureAwait(false);