page speed ninja Error message

Great Program but I’m getting the following 2 errors when I save my settings:

Warning: touch(): Unable to create file /var/www/html/wp-content/plugins/psn-pagespeed-ninja/cache/pagecache.stamp because Permission denied in /var/www/html/wp-content/plugins/psn-pagespeed-ninja/admin/class-pagespeedninja-admin.php on line 857

Warning: Cannot modify header information – headers already sent by (output started at /var/www/html/wp-content/plugins/psn-pagespeed-ninja/admin/class-pagespeedninja-admin.php:857) in /var/www/html/wp-includes/pluggable.php on line 1219

The page I need help with: [log in to see the link]