Try:
cd /var/tmp
chmod +x sng_freepbx_debian_install.sh
sudo ./sng_freepbx_debian_install.sh
If that doesn’t work, then maybe:
cp /var/tmp/sng_freepbx_debian_install.sh /home/sangoma
cd /home/sangoma
chmod +x sng_freepbx_debian_install.sh
sudo ./sng_freepbx_debian_install.sh