You can update this value with the command:
# mysql -uadmin -p`cat /etc/psa/.psa.shadow` -D psa -e 'update ServiceNodeEnvironment set value="true" where name="isHardQuotaEnabled"'
Here is the list of commands for Plesk Onyx and Ubuntu 16.04:
apt install plesk-php71-dev gcc make git pkg-config zlib1g-dev libmemcached11 libmemcached-dev
git clone https://github.com/php-memcached-dev/php-memcached.git
cd php-memcached/
git checkout php7
/opt/plesk/php/7.1/bin/phpize...
Just tested it now. All works fine with 16.04 and php7.1, not sure that it works, but it is installed and enabled in Plesk now!
There were some dependencies and some other package names:
apt install plesk-php71-dev pkg-config zlib1g-dev libmemcached-dev
P.S. read also an article about server...