According to the pop-up message, your issue may relate to the difference in Apple ID for signing up and downloading.
Please make sure that you signed up subscription by the Apple ID that you downloaded the app. To make it easier, please follow these steps.
1. Delete the app
2. Log in iTunes and AppStore by Apple ID that you used for signing up the app.
3. Reinstall the app
4. Tap Restore Purchase in Setting. (Make sure you are connecting to a stable internet connection/wifi when you try to restore purchase). You can restore those purchases with your Apple ID login credentials without having to pay again.
5. Try to use Premium features.
If it doesn't work, please try to restart your device and try again.