If you're trying to access TextNow from a rooted or emulated device, you might see this error:
TextNow can only be accessed on mobile by non-rooted, physical, compatible devices: Android smartphones, iPhones, and 3G/4G data-enabled tablets. It cannot be accessed by computer-based Android or iPhone emulators, or devices that have been given root access (i.e. "rooted"), or have had their operating software modded or otherwise modified.
If you have a used phone that you didn't purchase from TextNow, it might be rooted without your knowledge. If you have an Android device, you can run a quick test by downloading an app called SafetyNet Helper - don't worry, it's completely free. If you run that app and see a big green check mark, your device is not rooted. If you see a big red X, it's a rooted device.