Whew! That is a good question! And not one that I can give you a direct, affirmative answer to. You are not going to get in trouble for posting links to other on-platform content. In fact YouTube rewards you for doing so, though you don't get to see this. We all worry about watch time, right? What you don't see is studio watch time. For example, if you make a five minute video with an end screen that leads to another five minute video and both get 100% AVD then that first video gets an AVD of 5 minutes... but it gets credit for 10 minutes worth of studio watch time. This is a VERY generic representation of how that works, but it explains the importance of getting viewers to watch until the end of the video.
So a YouTube link is not going to hurt you. What does hurt you is that those first 200 characters are prime real estate for meta data... you really want to put as many of your most high-value keywords in there as you can fit naturally. A YT link takes up a lot of room... room that could go to keywords. The link is much better placed after the initial video description.
Unless...
Ok so there is one other thing that creators will do which throws this off but is worth mentioning. You can put a YouTube link in the very top of the description. I don't like this idea, but there is some reason for it. The majority of viewers do not click on the description to read it, they simply see the first 140 characters which are shown just beneath the actual video (when viewed on mobile). That being the case this gives them a change to click that link because it is right there in their face. It's a link that they must pass in order to write a comment (and you should also have a pinned comment with a link to another video). The idea here is that you want to get as many opportunities for these viewers to click on a link and watch another video as possible.