You can have a default administrator as the signatory but if they are not available for signing then you can change it to another administrator
If you want to change the signatory for signing, you can follow these steps.
Go to the menu selection: 'Tasks' -> the sub heading 'TASKS' -> select the document for which you wish to change the signatory -> press the action button (three dots) on the right hand side and select 'Change Signatory'
You will then be brought to this page:
If you don't have another signatory to replace the current one, you can modify admin access by selecting the green 'Configure Signatories' button. Which will bring you to the 'User Access' page where you can change signing authorisation.
If you wish to change the default signatory, this support article link can offer guidance: Default Signatory