From what I can see on the video you uploaded you appear to be trying to adjust the corner at the end of a line.
In the video when you first started to draw, it appears you were about to create a ‘V’ shape but the second part of the line never gets completed. You then are trying to adjust the corner at the ends of a straight line which you can’t do. I
f you had drawn the complete V shape and then, sub selected the middle point and adjusted the corner for that, it should work fine
Hope this helps