The 'cannot open .git/FETCH_HEAD: Permission denied' error usually signals restricted permissions on the .git directory, which Git needs access to for operations like fetch or pull. Start by checking permissions using `ls -la .git` to ensure your user account has 'read' and 'write' permissions. If not, adjust them with `chmod -R 755 .git`. For Windows, right-click the .git folder, go to Properties > Security, and make sure your user has full access. If permissions look correct but errors persist, try running Git commands as an administrator (`sudo git fetch` on Unix, or 'Run as Administrator' on Windows). Resolving these permission issues typically allows Git to execute commands without obstruction.