Contao is not allowed to edit files on your server. Activate the Safe Mode Hack to allow file operations via FTP by adding the following lines to the local configuration file system/config/localconfig.php:

$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 try to fix the issue by changing any file permissions! If there are any file permission issues, always use the Safe Mode Hack.