There must be mailbox numbers available for new renters choosing to sign up online. This is how you "reserve" mailbox numbers for these new renters, effectively giving them a list of box numbers to choose from.
Click Mailroom from the top menu
Click Mailboxes from the sidebar
Check the box to the left of the mailboxes you wish to reserve for online signup
Click the ... icon from the submenu and select Reserve
It is always safe to reserve too many mailboxes for online signup than not enough.