Can I purchase an SSL certificate for more than one year?

As of August 14, 2020, it’s no longer possible to purchase certificates for a period of longer than one year. As explained in this article, with ballot SC31, the CA/B forum has effectively prohibited Certificate Authorities from issuing certificates that are valid for longer than 397 days, effective September 1, 2020.

This is an industry-standard requirement. It applies to any Certificate Authority or reseller, and not just to DNSimple.