That looks like a Caddy v1 config. Please upgrade to Caddy v2. Caddy v1 is now EOL (no longer maintained).
If you make a site block with only domain.com, Caddy will only issue a certificate for domain.com. If you want a certificate for www.domain.com as well, then specify both, separated by a comma:
Caddy can issue wildcard certificates if you need it, but it involves extra work, i.e. enabling the ACME DNS challenge: