This can happen if:
- your network does not permit your device to connect to the google play license servers. This is rare but some people have network firewalls in place to prevent such connections.
- the license servers have not been updated (this is rare because CC waits for an hour before checking the license)
- your identity set in the play store on the device is not the same as the one used to purchase CC. This is the most common reason for license failures.
- the device is a kindle or (rare) some other device that doesn't have the google play services installed. The Boyue T62 is example or a device that is shipped without google services. In this case you must manually install the google play services before CC will successfully license.
EDIT: Looking at your other thread "how do I install", you mention that CC is not in the "My Apps" list on your device. This is a very strong indication that the account set up for google play on the device is not the account you used to buy the app.