Originally Posted by BenA

It’s likely that your flight simply doesn’t have any upgrade inventory available. If it’s more than 24 hours before departure, try calling again and asking to waitlist for the flight - the agents should be able to do that without inventory being available, and then your upgrade will go to the gate if it doesn’t clear. If the agent objects again saying GUCs can’t be applied to A350 flights, try pointing them at this thread and asking them to escalate to figure out what’s going on internally...