For Debian Stretch users it is required to install official MySQL from Oracle’s repos first, before installing Bluecherry.  MariaDB provided by default in Debian repos as a replacement for MySQL is  not compatible with Bluecherry.

Please follow the instructions in this link to install Oracle MySQL

# echo -e "deb stretch mysql-5.7\ndeb-src stretch mysql-5.7" > /etc/apt/sources.list.d/mysql.list
# wget -O /tmp/RPM-GPG-KEY-mysql
# apt-key add /tmp/RPM-GPG-KEY-mysql
# apt update
# apt install mysql-server

After that, use these command to install Bluecherry

wget -q -O- | sudo apt-key add -
sudo wget --output-document=/etc/apt/sources.list.d/bluecherry-stretch.list
sudo apt-get -y update
sudo apt-get -y install bluecherry