Thanks. I installed v20 and it worked. By the way, I noticed that step 2 only accepts root as the username, the superuser username also has the same issue. I had to redo this step a couple of times before it worked. I guess there is a bug in the installer.
Well anyway, I am very interested in further developing Dolibarr. I know Python and some basic web page programming. Can you suggest some good resources for learning PHP and Dolibarr dependencies?