The instructions for " Monitoring VMWare ESXi" are well written from the Checkmk side. But there is a very non-intuitive menu step needed in the ESXi setup that would probably save users real headaches if it were mentioned here.
The documentation states: “You must have defined a user on the ESXi-server. It is sufficient that this user only has read access.” Well and good, but I only learned after much frustration that giving that user rights is done in a completely different place as user creation. (https://superuser.com/questions/1219405/creating-user-in-esxi-6-5-and-assigning-role) If you do not want to explain the proper steps or link to a page that does, you might give a hint, that this can be tested by seeing if the user can log into the ESXi GUI. If it cannot login, then Checkmk will not be able to monitor the host.