It is the host machine which needs the sshpass program installed. For an Ubuntu machine such as 16.04, it’s as simple as apt-get install sshpass. Again, this error message of:
ERROR! to use the ‘ssh’ connection type with passwords, you must install the sshpass program
Applies to the HOST (provisioner) not the GUEST (machine(s) being provisioned). Thus install sshpass on the provisioner.
For installing sshpass on Mac OS refer to this link