An update was made to the PBS Video app on Amazon Fire TV and Amazon Fire TV Stick requiring users to reactivate their PBS Video app. Click here to activate.

When I try to activate my Apple TV, I see a screen asking for a 4-digit code

You may have user restrictions implemented on your Apple TV. To deactivate or learn more about this setting click the following link: https://support.apple.com/en-us/HT200198