By default, only administrators can create repositories in Phabricator.
To allow users to create repositories, log in to Phabricator as an administrator, select “More Applications” in the left menu, find “Diffusion” in the application list and click “Configure” on the right opposite it, then click “Edit Policies” and for “Can Create Repositories” select “All Users”. Or select “Custom Policy …” and configure it according to your preferences.