Upgrade processing. The thing is broken and hiding the availability was their first choice.

Here is the fine print -

Not all upgrades are cleared in real time. Our process of clearing upgrades varies based on the type of upgrade and when the upgrade inventory becomes available. Upgrades that are not able to be cleared in advance will be managed at the departure gate, subject to availability and in priority order.

Complimentary Premier Upgrades (CPUs): As a Premier member awaiting a CPU, you may see upgrade availability and still not be cleared. This is because the process for clearing CPUs runs at 24-hour intervals before the departure date based on the eligibility window for your Premier status. Inside of 24 hours, the process will run more frequently. When these processes run, they take into consideration all customers awaiting an upgrade at that time and clear them in priority order.

All other MileagePlus upgrades: For all other MileagePlus upgrades, waitlists are cleared in real time except during the 24-hour period before trip departure. Inside this 24-hour window, upgrades requested for flights in CPU-eligible regions will be cleared using the same automated system that processes CPUs. For all other flights, upgrade clearance inside of 24 hours will be managed at the departure gate in priority order.

Our phone agents will not clear pending upgrades. Our representatives do not have complete visibility into the upgrade priority order for a given flight. Because we would not want to potentially clear customers in the wrong order, upgrades are only processed by our automated system, or at the gate prior to departure. You can view the status of your pending upgrade through Manage Reservations on united.com