Youtube is meant to remove video editing tools that are not very used by its users. The catalytic date appears to be September 20 2017.
Unless you are a frantic user YouTube, you may not be aware that the popular website has a video editor and photo slideshow for editing your videos in its tools. And maybe you're learning it a bit late, after Youtube decided to abolish them. If you have already tried it and you are in the middle of editing your video, you will have to finish it and publish it by 20 September, as says Google on the YouTube support page.
The possible cause of these tools being removed is, of course, because they do not have a demand, and that is why the process is extremely slow when it comes to processing one's own video locally on his own computer with his own programs. Editing via Youtube requires uploading your video to edit and download the final copy. That said, as an idea was a good choice for Chromebook users or kids with low-power laptops or tablets.
However, Google points out that many free and paid third-party processing tools are available. In other words, the app was useful as a way to attract YouTube customers when 2010 came in. But with so many applications out there, resources are no longer worth it.
If you are willing to pay and have a decent Intel or Mac machine, there are interesting autonomous applications for this job, such as Adobe Premiere CC, Allpe's Final Cut Pro X for Mac and Avid's professionally-oriented Media Composer. Avid also has a free application called Media Composer First, Apple has iMovie for Mac and Clips for iOS, while Microsoft has Windows Movie Maker. If you just want a web editor, there are options like Magisto and WeVideo that work in most browsers, including the Chrome browser on your Chromebook.
Finally, Google maintains Youtube enhancement tools, which can filter, mute and cut videos.