AWS Cognito - .NET Framework

I'm trying to connect to AWS Cognito from a .net Framework project.

I can sign up succesfully but when I need to sign in, throws me an error: "User pool {userPool} client does not exist".

This is my code:

private async Task<bool> CheckPasswordAsync(string userName, string password)
        {
            try
            {

                
                var authReq = new AdminInitiateAuthRequest
                {
                    UserPoolId = ConfigurationManager.AppSettings["USERPOOL_ID"],
                    ClientId = ConfigurationManager.AppSettings["CLIENT_ID"],
                    AuthFlow = AuthFlowType.ADMIN_NO_SRP_AUTH,
                    
                };
                authReq.AuthParameters.Add("USERNAME", userName);
                authReq.AuthParameters.Add("PASSWORD", password);
                

                AdminInitiateAuthResponse authResp = await _client.AdminInitiateAuthAsync(authReq);


                var auts = authResp;
                return true;
            }
            catch(Exception ex)
            {
                var exc = ex.Message;
                return false;
            }
        }

I don't know if I need to add more information to the method.

The user pool exists and is the same that I have in appSettings.

Any idea about what's wrong?

Thanks in advance.