The below from your MB's product page

Storage 5 x SATA3 6.0 Gb/s Connectors, support RAID (RAID 0, RAID 1, RAID 0+1, JBOD and RAID 5), NCQ, AHCI and Hot Plug 1 x M.2_SSD (NGFF) Socket 3, supports M.2 SATA3 6.0 Gb/s module and M.2 PCI Express module up to Gen2 x2 (10 Gb/s) (M.2_SSD (NGFF) Socket 3 is shared with the eSATA3 connector)

Best I can read from above and elsewhere on these pages is your fastest option will be an AIC card plugged into a PCIe slot as it appears the M.2 position is limited to SATA III speeds.

As you're located in the US contact ASRock Tech Support using one of the below means and verify first though as I'm not 100% convinced I'm correct.

Too, remember they may be on an extended vacation from yesterday thru Sunday due to Thanksgiving here.

Support Phone: 1-909-590-8308
Support Email: support@asrockamerica.com


And reply back with their answer if you remember please. Your MB fell smack dab in the middle of M.2 and NVME tech being introduced so I'd personally as a Moderator here like to know for future reference if you'd be so kind.