Although logged in as site admin I see this when accessing pending store vendor page. Vendors are setup to need approval first. Only seen after adding groups addon. “Notifications: You don’t have permission to access this page. Please contact your Store Admin for assistance.”
Did you mean to say that you can’t access the pending vendors list page where admin will approve them? If yes then please check whether you are using any User Role Editor plugin or not and if it exists then do deactivate it and check again.