It is true that a Head Office Account cannot be retired, so the solution lies in making the account no longer a Head Office Account. The easiest way to do this is to navigate to the Account Hierarchy tab of the account you wish to retire, here you will see that the account is the Head Office for the child accounts in the list.
To unlink these accounts to be able to remove Head Office Account status:
Right-click the Child account in the Account Hierarchy list and select Locate to this Account
Now that you are on the Child accounts settings go the Credit Control tab, and select the Relationships tab
The field called Head Office Account contains the Account Number for the account you want to retire, remove this reference or replace it with another account.
Repeat the above steps for all linked Child accounts until there are none left, once there are none left the system will remove the Head Office flag against the account you wish to retire thus making it possible for that account to be retired.
To retire the account, please read our guide on How to Retire an Account