I get "Upload directory is not writable" error when uploading media.

This issue is typically caused by a wrong configuration for the 'tmp' folder in the Joomla configuration.php file. This folder is used to temporarily store the uploaded media for further processing.

This can happen if you moved your site to a different folder or a different host. Whenever you do that, you need to edit the configuration.php file and update the path to the tmp folder.