There is a lot of conflicting information about whether hardware encryption for Bitlocker works on the older XPS 15 laptops (9550 and probably 9560).
The answer is no, it does not work. The BIOS does not implement the required UEFI features. It fails when rebooting to apply, and when skipping the hardware test, soft-bricks the SSD.
Note I am referring specifically to Bitlocker EDrive mode (hardware encryption). Class 0 encryption also does not work on these laptops as there is no facility to set a BIOS password for NVMe disks.