snaps are only available from Debian 9 onwards.
Debian 8 is end of life and is no longer supported by the Certbot project. I think there's probably no way to fix this unless you upgrade to Debian 9 (at least).
If you need a certificate urgently, you might have luck with one of the other client options like acme.sh, lego, etc.
Edit: you might still be able to scrape through by using certbot-auto instead of certbot from apt. But this isn't officially supported for Debian 8 and will soon be disappearing.