Unable to play “For All Mankind “ but can watch other shows such as “Lewis”
‘Lewis’ (Inspector Lewis?) is not on Apple TV+, so that may behave differently. At least they are independent from each other.
Can you confirm that you have an Apple TV+ subscription linked to the Apple ID account with which you are signed in to the Apple TV app? See your subscriptions overview - Apple.
on Roku
Another user (mygreensocks) suggested this procedure from Roku Support staff, which reportedly helped several users.
1. Remove the Apple TV app,
2. Shut off and unplug the TV (or streaming device),
3. Wait 30 seconds,
4. Plug the TV (or streaming device) back in,
5. Reload the Apple TV app.
Else try the Network Connection Reset, as mentioned by user Jarieta.
If the issue persists after that, then please contact the Apple Support staff for further assistance.