"This TV show is not that interesting."
The options in Japanese could only say "this television is not that interesting". Show is ばんぐみ
です is not actually needed for い-adjectives, it just makes the sentence polite.
The problem is that the duolingo course is really inconsistent in its usage of polite and informal speech, and doesn't explain at all.
What's annoying though is that if you leave it out other times, it marks it as wrong! So it's fine if it isn't needed here, it should also be an option to leave it out on other questions too!
It accepts without the の now, at least: このテレビ番組はあまり面白くないです。I'm a beginner, but seems like the course mods are really trying to drive home the message about not getting into the habit of using テレビの番組 and アメリカの旅行. :P