Hi @charliecobb,
Welcome to Box Community and glad to help!
If your account is at its purchased seat limit, adding another managed user triggers an in‑app prompt to add a seat and pay for it. Box has minimums and billing rules for seat counts and prorated charges.
Upon checking your current seats on your account : 3/3 (used / available). You may check your plan and current seats: Admin Console → Account & Billing → Seats Used.
Confirm whether you’re adding the MD as a managed user (paid seat) vs inviting them as an external collaborator. If you only need them to view/comment, invite as a collaborator or share a link instead (that may avoid adding a managed user) I Have More Users Than Seats Error
If you need them as a managed user but don’t want a charge and you believe the portal is incorrect, open a Billing ticket or contact billing@box.com / support via Admin Console or you may reach out to our Billing Support by contacting them directly by emailing billing@box.com or by phone at 1-877-729-4269 and follow the prompts for existing customers.
Hope it helps!