How can I troubleshoot issues with package repositories?

Verify that the repository URLs in `/etc/apt/sources.list` and the files in `/etc/apt/sources.list.d/` are correct and up-to-date. Check for typos or outdated repositories. You can use `sudo apt update` to refresh the package lists and see if any errors occur related to repository access. If you recently added a repository, ensure it's compatible with your system and that the GPG key for the repository is properly installed.