If you are facing the same problem as stated above, there may be some issue related to path of docker-machine location and virtualbox location.
When docker installed, its path is like /usr/snap/docker and virtualbox is /usr/bin .
Dockermachine is unable to resolve using absolute path. It is searching in relative path( possibly a bug)
I reinstalled docker.io using ( apt install docker.io ) and did custom setup for docker machine for ubuntu 18. Finally I have all executable in nearly same path
I think, this will help you to resolve problem by some extent.