about more information that you requested for this issue, we explained everything in question but I can provide you the source code: 

AppName = "Ino Pc Agent"
AppId = "9C3EF09C-DA54-4531-9A82-8F1EEA922AE0";
TaxYear = "2021"

mentioned properties(AppName, AppId & Tax Year) are added manually for testing purposes.

public void Authentication()
{
     ILacerteAccess lacerteAccess = new LacerteAccess();
     ITPRegistration tPRegistration = lacerteAccess.CreateTPRegister();
     TPSDKStatus Access = tPRegistration.Register(AppName, AppId, TaxYear);

     if (Access == TPSDKStatus.AccessGranted)
     {
          MessageBox.Show("Access Granted");
     }
}

We encounter LacerteSDK.SharedInterfaces.TPSDKStatus.LacerteTaxSetupRequired error when we want to register the app by using tPRegistration.Register using same mentioned properties(AppName, AppId, TaxYear).

if you need more information could you specify what exactly you need.