Any new iPhone or iPad purchased after September 10, 2019 is eligible. Mine is an iPhone 11 and I was able to sign up for the one year offer. This instruction worked for me:
Open the Apple TV app and sign in. Then go,to,your computer and open a desktop web browser, go to https://tv.apple.com and log in using your Apple ID credentials. You should be able to accept the 1-year free trial offer from there. Once you have accepted the offer on your computer, go to your iPhone, sign out of the TV app, close it, then launch the app and sign in again.