What do you mean you can't post Youtube videos? You don't actually post the video itself, you know. You just need to post the link to the video. When you go to Youtube, the link is at the top of the screen.
It's going to look something like: [
www.youtube.com] with some more letters and numbers at the end
After you post the link, it automatically opens up the video. This is what works for me when I post a video using my desktop. I never try it from my iPhone. That may be a horse of a different color.