Installation and everything went smoothly for my first wordpress multisite. Issue I came across and did step by step is:

Installed Theme > Network Activated
Created a new site >

Logged in as the demo user to activate the theme and I get the following message:

You attempted to access the "A WordPress Site" dashboard, but you do not currently have privileges on this site. If you believe you should be able to access the "A WordPress Site" dashboard, please contact your network administrator.

If you reached this screen by accident and meant to visit one of your own sites, here are some shortcuts to help you find your way.

Weird thing is too obviously when I click the Visit Dashboard link (as demo user) I get the same message, which I understand, because "demo" isn't the network administrator.