What common file permission issues prevent `debug.log` from being created or written to?

The most common file permission issue is when the web server user (e.g., `www-data`, `apache`, `nginx`) does not have sufficient write permissions for the `wp-content` directory or the `debug.log` file itself. Permissions for `wp-content` should generally be 755 (owner can read/write/execute, group and others can read/execute). If the `debug.log` file already exists, its permissions should also allow the web server user to write to it (e.g., 644 or 664). Incorrect ownership (where the web server user is not the owner or part of the group with write access) can also be a factor.