How to restore administration account on Imac

There are several ways to do this. You can remove flag file for the setup process and it'll loop you back into the out of box setup process and let you create a new Administrator account. I would backup any data before attempting this. You're going to be required to access the recovery environment and accomplish via Terminal, or via Single User mode and Terminal. There will be additional steps if you have a new system with a T2 security chip. 

We do apologize for the delay in answering your post. If you want directions and to attempt we can certainly provide information for you. This is something I would recommend bringing in though.

This is something that we can try to look at in-store at any location.  An appointment is not needed for service, there may be some available online at https://www.microcenter.com/site/service/service.aspx Otherwise please visit the store with the computer and charger/power cable for assistance, store hours are 10am until 9pm Mon-Sat, 11am-6pm Sun.