If you have a current subscription, pressing Restore  should activate the subscription. 

(if you are a tester as in you install ezccs from TestFlight click here)

Apple tracks your subscription using your Apple ID. I have no access to subscription information.  ( if you are using the app on a device that doesn't use the Apple ID with a subscription, perhaps a United iPad,  see the FAQ titled United iPads)Check you have the current Apple iOS for your device.  Apple has been updating frequently.  Look under Settings->General->Software Updates.  Having the current/correct iOS resolves most subscription issues.

Troubleshooting:

The first thing to try is double touch the home button on the device and reboot the device.

Next verify your subscription using this:

https://support.apple.com/en-us/HT202039

If you have a current subscription, pressing Restore  should activate the subscription.

If you have in app purchases restrictions turned on this can block the subscription.

https://support.apple.com/en-us/HT204396

I have noticed that a bad data connection can also cause issue with the subscription. You might try on WiFi. 

In some cases it is necessary to delete the app and re-install it.   

(Users have been ignoring the advice below, much to everyones discomfort.  The subscription system is not mine, it's Apple's and sometimes it just needs a reset.  please do not ignore the steps in this FAQ.  Each steps is here for a reason.)

In some cases you need to sign out of your Apple account and sign back in.  Settings->iTunes & App Store.. 

Turning off Screen Time may also fix a Subscription issue.  Specifically when in-app purchases are blocked.