File System Permissions

One common reason for this error is incorrect file system permissions.

Magento requires specific permissions for directories and files to function properly.

Incorrect ownership or permission settings can lead to a 'Permission Denied' error.

Ensure that the Magento files and directories have the appropriate ownership and permissions.

You can recursively set the correct permissions using the following commands:

sudo find var generated vendor pub/static pub/media app/etc -type f -exec chmod g+w {} + && sudo find var generated vendor pub/static pub/media app/etc -type d -exec chmod g+ws {} + && sudo chown -R :<your_webserver_user> . && sudo chmod u+x bin/magento

where  should be replaced with the actual web server user, such as 'www-data' for Nginx.

SELinux or AppArmor

Another possible cause of the 'Permission Denied' error is the presence of Security-Enhanced Linux (SELinux) or AppArmor.

These security modules may interfere with Magento's file access.

If your server employs SELinux or AppArmor, ensure that the policies allow Magento's operations and file access.

Analyzing Logs

Check the specific log files for your web server and PHP to identify any errors or permission denials.

The web server log (e.g., Nginx's error log) and PHP-FPM log can provide valuable information on the cause of the 'Permission Denied' error.

Troubleshooting Tools

Utilize tools like strace and lsof to trace file system and process activities.

These tools can provide insights into the precise file or process causing the permission denial.