The "An error occurred in the upload. Please try again later" message is a common issue encountered by WordPress users, often due to incorrect file permissions, low PHP memory limit, large file size limits, or incompatible plugins. To resolve this issue, one can check and adjust file permissions using an FTP client, increase the PHP memory limit by modifying the wp-config.php file, update the file size limit in the functions.php file, disable plugins one at a time to identify the problematic plugin, and keep the WordPress installation up-to-date.