[bootstrap] Drupal 9 Starterkit not compatible

Firstly, forgive my ignorance if that's the only problem. I'm more of a design person and much less so on the developer spectrum.

I did a fresh install of Drupal 9 and am trying to create a sub-theme using the included starterkit. When I try to install it, I see the following message: "This theme is not compatible with Drupal 9.0.0. Check that the .info.yml file contains a compatible 'core' or 'core_version_requirement' value."

Am I doing something wrong? Thank you!