The 'fatal: repository not found' error often occurs when there is an issue with the repository URL or if you lack the necessary permissions. Start by double-checking the URL in your `git clone` command to ensure it matches the repository’s URL exactly, including capitalization and syntax, as GitHub URLs are case-sensitive. If you’re cloning from a private repository, ensure you have permission to access it, either through collaboration permissions or by using a personal access token (PAT) in place of a password. For example, use the URL format `https://username:[email protected]/username/repo` when cloning a private repository over HTTPS. Additionally, confirm the repository still exists and hasn’t been deleted or made private by the owner. Ensuring correct URL syntax and permissions should enable a successful clone.