На SetUp должно быть («имя платформы», «iOS»)
public void StartDriver()
{
DesiredCapabilities cap = new DesiredCapabilities();
cap.SetCapability("platformName", "iOS");
cap.SetCapability("deviceName", "iPhone Xr");
cap.SetCapability("automationName", "XCUITest");
cap.SetCapability("app","YourApp.app");
cap.SetCapability("autoAcceptAlerts", true);
driver = new IOSDriver<IWebElement>(new Uri("http://127.0.0.1:4723/wd/hub"), cap, TimeSpan.FromSeconds(300));
Assert.IsNotNull(driver.Context);
}