Hmmm, turns out it isn't supported on 19.04:
error executing enable: Livepatchd error: The platform Ubuntu 19.04 is not supported. exiting.
I just set up a 19.04 machine and have not seen that notice, so perhaps it's not an issue? Or perhaps it is only for LTS versions...
Sorry, it is only for LTS versions. When you sign up for an account it says LTS only.