Ubuntu – Ubuntu 16.04 installer can’t find the SSD on the Dell XPS 15

1. In Windows, run Command Prompt as admin
2. Invoke a Safe Mode boot with the command: bcdedit /set {current} safeboot minimal
3. Restart the PC and enter your BIOS during bootup.
4. Change from IDE to AHCI mode then Save & Exit.
5. Windows 10 will launch in Safe Mode.
6. Right click the Window icon and select to run the Command Prompt in Admin mode from among the various options.
7. Cancel Safe Mode booting with the command: bcdedit /deletevalue {current} safeboot
8. Restart your PC once more and this time it will boot up normally but with AHCI mode activated.

(Source)

From the Arch Wiki for this device:

When the SATA-controller is set to "RAID On" in Bios, the hard disk (at least the SSD) is not recognized. Set to "Off" or "AHCI" before attempting to install Arch. If dual boot to Windows is intended, follow 1 to work around the "INACCESSIBLE_BOOT_DEVICE" error.

The same goes for an Ubuntu install, most likely. Switch the SATA controller in BIOS to 'AHCI' or 'Off' before trying to install Ubuntu.

If this doesn't work alone, try modprobe nvme to load the necessary driver for the NVMe SSD.