Your caption shortcode seems to be missing the most important part: the image.
The caption shortcode in WordPress usually looks like this:
<img src="http://localhost/wp-content/uploads/2010/07/800px-Great_Wave_off_Kanagawa2-300x205.jpg" alt="Kanagawa" width="300" height="205" class="size-medium wp-image-6" /> The Great Wave
There you can see the alt attribute, which is automatically set when you enter the alt text after uploading an image (here: “Kanagawa”). The alt text is not visible to users, but to programs like screen readers. The other text inside the shortcode (“The Great Wave”) is the caption, which on the front end is displayed below the image.
If your theme does something completely different with the caption shortcode, you should contact the theme developer and ask them for help instead.