I'll try this myself... I do know adding repository on Debian is different. Can you verify you have PHP7.1+ installed?
apt install apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list
apt update
To add PHP7.2
UPDATE
Here is a copy of my 'history' while installing this just now on Debian 9.
sudo apt-get update
sudo apt-get install apache2 mysql-server mysql-client composer openssl
apt install apt-transport-https lsb-release ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list
apt update
sudo apt install -y php7.2 libapache2-mod-php7.2 php7.2-common php7.2-mbstring php7.2-xmlrpc php7.2-soap php7.2-gd php7.2-xml php7.2-mysql php7.2-cli php7.0-mcrypt php7.2-zip php7.2-curl php7.2-dom composer openssl
cd /var/www/
ls
cd html
sudo rm index.html
composer create-project flarum/flarum . --stability=beta
sudo systemctl restart apache2
sudo chown -R www-data:www-data ../html/
history
You can see it works here: http://159.65.226.233/