sudo apt-get install php7.1-phalcon
sudo: unable to resolve host official
Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package php7.1-phalcon
E: Couldn’t find any package by glob ‘php7.1-phalcon’
E: Couldn’t find any package by regex ‘php7.1-phalcon’ 

Solution:
sudo apt-get install curl
curl -s https://packagecloud.io/install/repositories/phalcon/stable/script.deb.sh | sudo bash
sudo apt-get install php7.1-phalcon