Only another Account Administrator would be able to grant you the Account Administrator Role. If you are already an Admin you would be able to identify who the Account Admin is by going to Admin > Team > Agents and the profile that you cannot edit would be the Account Admin's.
The Account Admin would be able to give you this by navigating to the Admin > Team > Agents >Edit, and if you're able to scroll further you would be able to assign roles to the agent. Please ensure you're given only the Account Administrator role and remove any other role if assigned.
Also, If that person is currently not associated with the company, please send an email to support@freshdesk.com with the person added in the CC so that we could do the needful for you.