Do you get the message “subscription service not available” when you want to start a subscription in the app? This message comes from Apple and can have several causes:
In-app purchases are disabled
Go to: Settings > Screen Time > Content & Privacy Restrictions.
You can disable this completely, or go deeper into the settings:
Then choose: iTunes & App Store Purchases > In-app Purchases > Allow
No connection with the app store
Apple returns an error or your phone is not successfully connecting to the services at Apple. This could be due to an unstable internet connection. You can try switching between WiFi and 4G to see if this improves things. Then press the ‘try again’ button (in the latest version of the app).