Link doesn't appear in configuration
system.admin_config is the top-level page at /admin/config. Normally you would not put your configuration link there - you would instead put it into one of the blocks that appear on that page. For example, the "People" block would be user.admin_link, so if you want your config under "People" use user.admin_link as your parent. Or if you want to put your configuration link under the "Development" block use system.admin_config_development as your parent.

If you want to create your own top-level block then you need to do things a little differently - see the core user module and how it defines user.admin_link in user.links.menu.yml and user.routing.yml - this shows how a new block is created on the config page. You can then put your configuration link in that new block.