TYPOlight is not allowed to edit files on your server due to safe_mode or file permission restrictions. You have to use the Safe Mode Hack to allow file operations via FTP. To enable the Safe Mode Hack, open system/config/localconfig.php and add the following lines:

$GLOBALS['TL_CONFIG']['useFTP'] = true;
$GLOBALS['TL_CONFIG']['ftpHost'] = '';  // FTP host
$GLOBALS['TL_CONFIG']['ftpPath'] = '';  // FTP path (e.g. html/)
$GLOBALS['TL_CONFIG']['ftpUser'] = '';  // FTP username
$GLOBALS['TL_CONFIG']['ftpPass'] = '';  // FTP password

Do not mess with file permissions! TYPOlight will not work correctly if you just make folders writeable. If there are any access restrictions, always use the Safe Mode Hack.