I’ve got the same error message when I used the acme2 installation where I did my tests before. So I made a fresh installation of acme.sh, then it worked as said
$ git clone https://github.com/Neilpang/acme.sh.git
$ cd .acme.sh
$ ./acme.sh --install
$ . ~/.bashrc
$ export NSUPDATE_SERVER=<IP_of_DNS_Server>
$ export NSUPDATE_KEY=$HOME/example.com.dnskey
$ acme.sh --issue -d example.com -d *.example.com --dns dns_nsupdate --dnssleep 30
This should do the trick. My DNS is a BIND so I’ve to use dns_nsupdate, maybe you have to use another update method for your DNS.