The 'Username not recognized' error during GitHub authentication on the command line often results from an incorrect username or outdated credentials. Ensure that the username you’re entering matches your GitHub account’s exact username, which is case-sensitive. If you’ve recently enabled two-factor authentication, a personal access token (PAT) replaces your password. Create a PAT under *Settings > Developer settings > Personal access tokens* on GitHub, and use it as your password for HTTPS Git operations. If you’re using SSH, confirm that your keys are correctly set up and linked to your GitHub account. This setup should allow the command line to recognize your credentials, enabling successful authentication.