Trying to install Drupal modules and themes: keep getting error "does not contain any .info.yml files."
TL;DR you're trying to install a theme or module in Drupal 8, which doesn't contain the required files. You might be trying to install a Drupal 7 (or earlier) module by mistake, or it may not be a Drupal module at all.

The 8.x-dev branch of the Beale Street theme is most definitely not Drupal 8 compatible. It's a straight copy of the current 7.x-dev branch, so it hasn't been converted yet. The maintainer has said as much.

Converting a module/theme from 7.x to 8.x requires a lot more work than just adding a yaml file, and if you don't find one in a particular release you can safely assume it hasn't been converted yet.

There are lists of modules and themes on Drupal.org that are tagged with 8.x. As you've seen, not all of them are ready yet. Bear in mind quite a few will still be in -dev, so they're not expected to be useable.