WordPress.com and WordPress.org are completely entirely separate.
WordPress.com is a product of the company Automattic, Inc.
WordPress.org is the site of WordPress, a blogging software which is built and supported by volunteers, and can be installed on hosting providers or used via Automattic's WordPress.com product.
You need to log in to your WordPress.com account at https://wordpress.com/ and then follow the guide at https://en.support.wordpress.com/domains/map-existing-domain/