To create your first Drupal 9 theme, I recommend using the 9-step guide laid out by specbee, with a single edit to make the project work properly. On step 7, we are told to create templates/system directories, and create a page.html.twig file within the system subdirectory. This will lead to an error without feedback from the program, and your theme will not display properly.
Instead of creating a templates/system directory tree, create a templates/layout directory tree. So replace templates/system with templates/layout.