When you paste the video link into the upload video bar, remove the "s" from https. That will embed the video in your post.
So instead of this
https://youtu.be/6HtQmzh4BsM
It should look like this
http://youtu.be/6HtQmzh4BsM
So instead of this
https://youtu.be/6HtQmzh4BsM
It should look like this
http://youtu.be/6HtQmzh4BsM