using NUnit.Framework;
using Xamarin.UITest;
using Xamarin.UITest.Android;

namespace AAP.UITests
{
    [TestFixture(Platform.Android)]

    public class TestBase
    {
        protected AppManager appm;
        protected IApp app => AppManager.App;
        protected AndroidApp androidApp;
        protected bool OnAndroid => AppManager.Platform == Platform.Android;
        protected bool OniOS => AppManager.Platform == Platform.iOS;

        public TestBase(Platform platform)
        {
            AppManager.Platform = platform;
        }

        public TestBase()
            { }

        [OneTimeSetUp]

        public void BeforeEachTest()
        {
            AppManager.StartApp();
            appm = AppManager.Appm;

        }
    }
}

I am using:
SpecFlow, SpecFlow.Nunit, SpecFlow.Tools.MsBuild.Generation - 3.0.199
NUnit, NUnit3TestAdapter - 3.12.0
Xamarin.UITest 2.2.7