You need to install the correct driver for your AHCI storage controller before changing the mode in the BIOS.
If you don't do so, after switching the mode Windows cannot load the storage controller driver needed to access the boot volume. Because this driver is loaded before the Plug and Play Manager, Windows can't automatically detect or otherwise search for the correct driver.
Pre-installing the driver should solve this.
That said, you don't mention the reason you want to do this, but if it's for reason of performance, in my experience there's no difference between the two modes on Dell XPS machines.