The main institutional account holder has all the permissions - so it can add new staff accounts via "Accounts and Access". You can read more about staff accounts' creation on the relevant page.
Any other staff member can also become an account manager if they are granted permission to interact with "Accounts and Access". So first of all, make sure that you have the permission for “Creating accounts” enabled.
If you are still unable to create an account, please contact us via the on-site feedback tools and make sure to attach screenshots and retrace your steps to help us understand the situation better.