If you are unable to see the activation code or the activation code is not working then you can do these few things that can help you to troubleshoot.
Recheck your internet
Sometimes network connectivity can create problems with the activation code. If you don't have a proper network connection then try to connect with a stable internet connection.
Close the app
There may be a bug or some issue in the app. So you have to close the app and then open it again. Maybe other users reported the bug and TNT team fixed it.
Restart your device
All the activation code issues can be fixed by restarting the devices.
Go to the System ⇒ System Restart, then wait for the Roku to restart.
Once it becomes restarted, Open the TNT and request for the activation code again.
Similarly, you can do this for other devices such as Fire TV, Smart TVs, Apple TV, and many others.