[NAME] The problem could be that your M.2 SSD is a SATA SSD, not an NVMe SSD. The Setup and Specifications say that an M.2 2280 SSD should have an interface type of PCIe Gen4.0x4 NVMe, up to 64 Gbps.
I installed a WD SATA  III M.2 2280 in the empty slot and its not detecting there is a drive installed. Do the XPS 8950 require a specific <.2? WD Blue 3D NAND 250GB Internal PC SSD - SATA III 6 Gb/s, M.2 2280, Up to 550 MB/s - WDS250G2B0B That is the one i installed. Thanks Dave
XPS 8950, SATA M.2 compatibility