First, please check whether your subscription is active directly in your iTunes account. This link will show you how to do this: https://support.apple.com/en-us/HT202039
It is usually possible to restore the purchase by tapping on the Restore button on the main screen.
If it does not help, please check:
1. Make sure that your iOS is up to date: https://support.apple.com/en-us/HT204204
2. Double-check that you are logged in with the same account that you used to purchase the subscription. If you are using a different account, make sure that you have Family Sharing set up: https://support.apple.com/en-us/HT201088
While logged in with the account used to purchase the subscription, you might also want to go into Settings on your device, tap Subscriptions, and make sure that the "Share New Subscriptions" is set to on. Additionally, you will be able to see a list of your subscriptions there. You can then tap on the subscription and set "Share with Family" to on to give access to another account under your Family Sharing.
3. Please make sure that you are connected to the App Store and have internet access.
4. Do you use parental control on your device? Sometimes, this error can happen when it is switched on.
5. Double-check that your card on file is active.
6. If nothing helps, we recommend to try deleting the app, restarting the device, and reinstalling the app from the store. Then, using Restore button should help.
7. Unfortunately, once or twice we've seen that it simply did not work on customer's home wi-fi, but it worked without any problems on a different wi-fi network.