Could you indicate the URLs of the subtitling page, and of the
YouTube page, please?
However, in general: subtitles you make with Amara / Universal
Subtitles do not get added automatically on the video at its
original location. Once you have subtitled the video, you must
download the subtitles to your computer, then add them again to the
video at its original location.
And you can only do that if you uploaded the video yourself on
that location, and you are signed in.
As the tab for adding subtitles on YouTube has recently been
replaced by a silly 1-item drop list opening from a little triangle
on the right of the Annotations tab, I attach a screenshot of the
Spanish interface, presuming it's the one you use.