if you are using ubuntu:
# vi /etc/apparmor.d/usr.sbin.mysqld
add followings or what is shown in the check
/path/to/piwik/misc/assets/* rw,
/path/to/piwik/tmp/assets/* rw,
# /etc/init.d/apparmor restart
# vi /etc/apparmor.d/usr.sbin.mysqld
add followings or what is shown in the check
/path/to/piwik/misc/assets/* rw,
/path/to/piwik/tmp/assets/* rw,
# /etc/init.d/apparmor restart