If you want to create your own theme, rather than extend an existing theme.

Either manually create 5 files in a new folder in your themes folder:

• style.css
• functions.php
• block-templates/index.html
• index.php
• theme.json

or download theme-experiments and run the create-empty-theme.php routine.

or follow the instructions at

https://github.com/WordPress/gutenberg/blob/trunk/docs/how-to-guides/themes/create-block-theme.md

This used to work…

https://developer.wordpress.org/block-editor/how-to-guides/block-based-themes/

This does… but it’s out of date.