This sounds like the bugs introduced by Roku when they pushed out OS14. There is a workaround that works for some, but not others. For some it loads and you get picture, but no sound. Give it a try.
In the settings for the Roku device, go to Audio → Digital Output Format → Custom, then make sure Dolby Digital is selected for the Dolby section. Can leave DTS off. Then go back to the Tablo app and try again.
Tablo just announced a firmware update that is going out now. It will be followed shortly by an update to the Roku app. This hopefully will fix the problem.
Check out this thread for more.
https://community.tablotv.com/t/tablo-roku-app-4th-gen-update-v-0-8-900/39231