If you are only going to run 1 VM, why do you have to have a reservation set? Just turn off the reservation.
You may be getting the error because of the Admission Control Policy settings. There has to be enough memory to support the 490GB of VM memory, the huge amount of VM overhead required to support the large amount of memory, and whatever ESXi needs to support whatever features you are running. When you couple that with an overly restrictive Admission Control policy, you might run into problems migrating or powering on your VM.