This error is mainly due to improper file permissions as stated in the warning message, you will need to make sure of two things:
– Make sure WordPress folders and files have the correct permissions (755 for folders and 644 for the files in most servers) check this link for more information about file permissions in WordPress.
 – All your files and folders have the correct owner/group based on your server configuration (www-data:www-data for Apache web server on Ubuntu by default) you may need to check this with your web host.