Create Super Admin as SaaS Platform

Please read carefully to save your time and my time, this is a very easy task. I already install in a "sub.domain" a php script for a restaurant waiting list. What I need to fix and modify is as follow: no matter if I am logged as user or admin, random appear error "502 Bad gateway". Also I will need to modify a few text at the layout (CSS). I need to finish create SaaS. At this moment the admin (me) of this script can create users and admins and select privileges for roles. But, no matter how many users and/or admins I create, all users/admins will operate in the same restaurant and that is not what I need, that is not a SaaS. What I need is once I create an admin, that admin will have their own completely separate restaurant with all the native functions/features of this restaurant waiting script. As SaaS have to be. For the SaaS I have to be the super admin, can create admins and those admins will have their own separate restaurant waiting system with all the native functions, features of this script.

This job have to be complete in 4 business days. Yes, more jobs available if this task complete satisfactory.