New Multisite - Subdomains/Subdirectories


I am trying to make a new multisite, I did it following your guide on this link:

I did all the steps and it seems to have worked ok, however one of the steps was not available.
on this step: "The next step is to visit the WordPress admin screens and install Multisite.
1.In the WordPress admin, go to Tools >Network Setup. You’ll be prompted to choose subdomains or subdirectories for your installation: choose the one that works for your network."

On this step I received a message saying that since it is an existing site (is a new one, but I installed wp as indicated in the guide) I have no such choice (don't remember the exact text)

The fact is that all new sites are open under subdomains, and I would like it to open under subdirectories, is there a way to fix this?
Should I uninstall and install again?