It is common to have staff who work in the office or in an indirect way with clients.
For the Scheduler to consider the shift as booked (not pending), a shift does need a staff member and a client assigned to it.
In this situation, you can create a client named "Office" or "Admin", after your organisation or something that makes sense to your team.
You are then able to create the shift for your office staff and add the new client to the shift.
It's also recommended to add a new shift type of "Office" or "Admin" so it stands out in your roster and you can distinguish which shifts are support-related and which ones are office/admin related.
Additionally, you might want to add a "No Charge" price or rate, which will allow you to work out how much the office time costs the business.