On your computer or phone; login at the cable provider's site (check subscription if needed and/or logout/login if you're using the "remember me" option) then "change provider" on the NBC site and login to the provider. Return to the roku device and start watching shows.
The problem I had when getting this error was that my sling subscription was expired for a year.
Syfy works on my other tvs and I got this error on my new one.
These solutions (clearing cookies etc) all point to the issue being a cookie shared from the cable provider and used on NBC in order to confirm subscription to store that on a device's memory to check with nbc.
Long story short, clearing cookies forces a full login on the cable provider's site. Testing this theory, I cleared no cookies, logged in with sling and updated the subscription, then redid the provider info on nbc's site, and I was able to resume watching shows without touching the tv.
If these work the way I think they do, I could only imagine how exploitable it may be.