Hmm…
Here’s the “Share” link (broken) for the Indiana clip linked above:
https: // youtu.be/ by-14R0C0yE
Now I’m going to try posting it with &4 at the end, unbroken.
You have to put the https:// in front of course, or the software doesn’t know it’s a link.
Well, from my computer and browser view (YMMV), just copying the Share link and adding an &(number) works just fine. No need to assemble separate components.
Yeah, that doesn’t work: An error occurred. Sorry, you can’t embed media items in a post.
This is the Share link (broken): https: // youtu.be/ by-14R0C0yE
That’s what I copied and pasted.
Try it with &8 at the end (or some suitably high number). As pointed out above, if there is already a preview generated for it in Discourse, it will fail unless you add the &(number) forcing it to go out and get data to generate a new preview.
Just saw your edit. Did you mean you pasted the broken link in, or an unbroken lin? Also, my preview was generated with &4 at the end. Did you try &5?
OK, that worked. Thank you.
I put that URL in another window to see what it was, and I’ve seen it a few times already on this board, so that’s probably the issue.
Glad to help, but I credit @Dr.Strangelove for the solution.
Sort of worked. I edited to say ‘OK, that worked. Thank you,’ and it said An error occurred. Sorry, you can’t embed media items in a post. I had to put a 9 after it.
Can somebody save me from having to read 200 posts? Is there a reason why Discourse suddenly decided to make things difficult? Seems like a dick move.
There is a bug that now thinks if you embed a video, you’re uploading it into a thread. You’re not, obviously, but it assumes you are. We don’t allow that on this message board, so it blocks you.
If you post before the preview gets loaded, you’re able to get it to not realize it’s embedded before you submit and it allows it. If you edit the post, it won’t let you save because it thinks that embedded video is an upload. So you tweak it just enough that it has to reach out to YouTube again, and then before it loads the preview again save the edit. You’re constantly tricking it to allow you to do something it mistakenly thinks is prohibited.
It’s a really irritating bug.
Wow. I don’t know if I can be that quick.
It really is.
If on a laptop (and maybe a tablet?), you can close the Preview window to the right of the Reply window. Reply window is the one at left that you’re actually typing into.
Once you close the Preview window, there is no timing issue to be concerned about:
I think this issue doesn’t come up at all for users on mobile. Unsure if that’s 100% or not.
Oh it does. Trust me. 
It’s just that we can’t turn off the preview (because there is none), you need to be quick.
This is not my experience. On a mobile device without preview (seems to be limited to phones), you have unlimited time, just if you had the preview window hidden manually. This doesn’t bypass the problem where the video may have been posted already, however (including if you edited your own post).
Then you’re lucky. On an iPhone with Safari it’s a massive pain in the ass. But it’s not consistent.
If you read just one post in the thread, read this one (#170):
Strangely, I have no issue with YouTube links, but YouTube links with a time stamp on them fail.
https://youtu.be/hoe24aSvLtw
https://youtu.be/hoe24aSvLtw?t=32
The first one works fine, the latter errors out.
Test:
It means someone posted that exact URL (with timestamp) somewhere else. Try the trick with appending “&1”, “&2”, etc. (except not “&1” since I just used that).