Just spinned up a test machine here with debian 9, running on hetzner cloud, and was able to install hestia without any issue. So it seems to be a system specific issue on your side.
For further debug, can you manualy download and adjust the installer from https://github.com/hestiacp/hestiacp/blob/560be4481d30bdf5a6e8ba9be5f41c4112d6d440/install/hst-install-debian.sh#L875-L891
So it only contains:
apt-get install $software
Instead
apt-get -y install $software > /dev/null 2>&1 &
BACK_PID=$!
# Check if package installation is done, print a spinner
spin_i=1
while kill -0 $BACK_PID > /dev/null 2>&1 ; do
printf "\b${spinner:spin_i++%${#spinner}:1}"
sleep 0.5
done
# Do a blank echo to get the \n back
echo
# Check Installation result
wait $BACK_PID
check_result $? "apt-get install failed"
And give it again a run? This should show you more informations.