I recently installed a Samsung 960 Pro M.2 NVMe on the motherboard and migrated the OS from the existing HHD. I disconnected the HHD and the computer booted up and was operating normally with the M.2. However, when I went to install the Samsung M.2 NVMe controller and the Samsung Magician software, both programs indicated that the Samsung M.2 was not connected. I called Samsung and the technician informed me that the only time the system won't recognize the M.2 is if the BIOS is set to RAID. He informed me that the computer would probably not boot up if I reset the BIOS to AHCI and I would have to perform a clean install of the OS. Both circumstances were true. Dell ships their computers with the BIOS set to RAID if it has a conventional hard drive!
There are ways to switch from RAID to AHCI without reinstalling -- that said DO NOT try without a complete backup of the system (or that fallback image -- you can always revert to RAID if it fails to work).
Search this forum and you will find that folks had problems getting the Samsung 960 Pro M.2 to boot when set to AHCI. Not sure if this is a problem with Samsung firmware or Dell BIOS or a little of both.