Wildcards are not available yet - you have to wait a bit longer (end of February, I believe). See: Looking Forward to 2018 - Let's Encrypt
First, we’re planning to introduce an ACME v2 protocol API endpoint and support for wildcard certificates along with it. Wildcard certificates will be free and available globally just like our other certificates. We are planning to have a public test API endpoint up by January 4, and we’ve set a date for the full launch: Tuesday, February 27.
and https://letsencrypt.org/upcoming-features/:
ACME v2 API
ETA: February 27, 2018
Wildcard Certificates
ETA: February 27, 2018