The SE7520BD2V server board has one PCI-Express x8 slot that is actually is x4 signals routed to a x8 physical connector. The video adapter you indicate is a x16 card. You can't "down-plug" PCI-Express cards because an expansion card with a higher number of lanes (the "x" value) physically won't fit into an expansion slot with a lower number of lanes. For example, a x16 expansion card won't fit into a x8, x4, or x1 slot.
For more information on the SE7520BD2V, see the server board Technical Product Specification.