Fix error & create Super Admin

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". I need to modify a few text at the layout (CSS). Also I need to create a Super Admin. The Super Admin can create admins and those admins will have their own separate restaurant waiting system with all the native functions and all the native features already this script have.

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