Dell Precision 7730 No Bootable Devices after adding 2nd M.2 PCIe NVME SSD

Hi, I have just installed a 2nd SSD (TRANSCEND 110S M.2 PCIE NVME 512GB) in my 7730 and I now get a "No Bootable Devices" message on start-up. I have 3 spare slots with the M.2 PCIe NMME interface and I've tried it in all 3 slots but the same error appears. If I remove the 2nd SSD then the laptop boots normally from the 1st M.2 SSD which is a PM981 Samsung 1TB. I have bitlocker on and secure boot. Could anybody please tell me how I get the laptop to "see" the 2nd drive? I want it for additional storage only. Thank you.