New to this forum but I found a way to get it to work on my system. I recently added F1 TV to my Roku account and like everybody else got extremely frustrated with the inability to get access through Roku. Here is the solution for my system - not sure why it shouldn't work on anybody else's.
1. Go to F1TV on your PC and select My Subscription (I'd deleted mine but its still there)
2. Under devices select edit the Roku device that you want to watch on and delete it.
3. Delete F1 TV from the Roku device/TV in 2.
4. Reinstall F1 TV (you will need be asked for user name and password)
5. Start F1 TV - it worked for me.
6. Interestingly, even though the app is now on Version 2.9, when I reinstalled, it still gives me version 2.0 It worked for me.
I'd be interested to know if it works for anyone else!