The TBM module doesn't list blocks that aren't placed in any region. To use a block in the TBM configuration (/admin/structure/tb-megamenu), that you don't want to show somewhere else on the page, place it in a Region at /admin/structure/block that is not being printed by your theme.
This is described in the Documentation here at point 2: https://www.drupal.org/docs/contributed-modules/the-better-mega-menu/how... →
You could create a new region called "Hidden Blocks for TBM" or so, and simply don't add it to your TWIG template page.html.twig. For this, just add a new region to your THEMENAME.info.yml as described here: https://www.drupal.org/docs/develop/theming-drupal/adding-regions-to-a-t... →
Then clear the cache and go to /admin/structure/block to place the block in the new region.
Doing so, the block is made available to the TBM module.
I've encountered the same problem with version 3.0.0-alpha2 in Drupal 10.2.2.
Since I'm not really into building modules or patches, I'd highly appreciate it if someone could provide a patch for the latest version (2 and 3) of tb_megamenu.
Thanks!
Fixed it by creating a folder named "simpletest" in /web/sites. You need to have /web/sites/simpletest.