That annoyed me as well given the wording on the ZeroSSL site suggested one has to create an account which is not true. I had hit an error using DNS-01 at the time. They have an entirely different page for ACME clients but it is not or was not linked from anywhere on the main page.

If anyone else ran into that it's just a matter of adding

    --server letsencrypt

You can also permanently change your default to LE — acme.sh actually has instructions for doing so in their wiki.

I rather liked using ZeroSSL for a long time (perhaps just out of knee-jerk resistance to the “Just drink the Koolaid^W^W^Wuse Let’s Encrypt! C’mon man, everyone’s doing it!” nature of LE usage), but of late ZeroSSL has gotten so unreliable that I’ve rolled my eyes and started swapping things back to LE.