I've been searching but what I've found was always about uploading a video to Tumblr or about adding audio only on Tumblr. None of the results has a solution for my problem. I just want to display a video that's already uploaded on YouTube, not uploading it to Tumblr. How do I embed YouTube video on a Tumblr text post?
I think you misinterpret what the user interface is saying.
There are two options in the video interface.
- Embed a video
- Upload a video
You want option 1. If you want to add some text you can use the caption.
Another way to do this will be a regular "Add a Text" Post
From there you can use html and YouTube embed option (using the share link at the bottom of a YouTube video) to place it where ever you wish.
<p>Go for text</p> <iframe width="560" height="345" src="http://www.youtube.com/embed/IwX3FdwMCUs" frameborder="0"></iframe> <p>And here goes the rest.</p>