It seems YouTube is employing some rather aggressive tactics to prevent users from bypassing one of its main revenue streams. They're forcing users who use ad blockers to disable them, so they either do or they can't watch the video. You read that right.
Advertising on YouTube
The advertising business on media and online platforms is becoming increasingly complex, especially with users employing certain tricks to bypass ads whenever possible. One of the best-known is ad blockers, which are used not only on websites—where banners are disabled—but also on services like YouTube, where you have to watch at least one ad before playing almost any video.
Although the integration of these ads is up to the creator, we generally encounter two ads before each video —and perhaps a few more interspersed—which can sometimes detract from the experience. Therefore, many users opt to use ad blockers to skip these ads for their convenience.

But, of course, YouTube doesn't like that one bit. That's why they've set to work trying to fix it. Since they can't prevent people from using these workarounds, they've taken a hard line: either you disable it or there's no video.
In testing period
At the moment it is something in a testing period that YouTube is experimenting to study its final implementation. This is pointed out in the official YouTube subreddit, where it is indicated that some users are receiving a notice like the one you see below in which they are told that "ad blockers" are not allowed on YouTube -since they allow everyone to be able to watch videos for free and if what you want is not to see advertising, you have the option of signing up for YouTube Premium.

The company has confirmed to Gizmodo that they are indeed in the testing phase with this and defends itself by indicating that ad blocker detectors are by no means something new and are commonplace on the web, where visitors are using them more and more.
Therefore, everything seems to indicate that once this beta phase is over , YouTube will implement this new measure globally and across its service to prevent these workarounds. You'll then have no choice but to give in and disable the ad blocker, subscribe to Premium… or resign yourself to not seeing their videos.
[Via Geeknetic ]