Did you also install 2 CPUs as specified by supermicro
See here https://www.supermicro.com/products/motherboard/Xeon/C600/X9DRX_-F.cfm
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PCI-Express
10 PCI-E 3.0 x8 slots
1 PCI-E 2.0 x4 (in x8) slot
(Both CPUs need to be installed for full access to PCI-E slots and onboard controllers...