>>/2197/
I found an explanation.
https://roboin.io/article/en/2026/01/11/how-to-stack-images-vertically-on-x-and-common-issues-explained/

If it's not working, reason's why could include the following.
- You're not a paid user.
- You're trying to post from a mobile device.  (It seems to be a desktop-only capability.)