This error is usually caused by a motherboard BIOS that hasn't been fully updated. Fortunately, according to a Dell represenative, this can be resolved. Your computer will need to be using the latest BIOS revision in order to work properly with a GTX 900 series card. However, if you're still using an older revision of the BIOS, the same represenative has recommended to update incrementally, in order of release.

For the Dell XPS 8700, the link for the BIOS updates can be found here: https://downloads.dell.com/published/pages/xps-8700.html -- scroll down to the BIOS category, and apply the updates from A1 up to A10.

For the Dell XPS 8500, the link for the BIOS can be found here: https://downloads.dell.com/published/pages/xps-8500.html

For further information on this topic, please see the first post mentioned in this thread: https://en.community.dell.com/support-forums/desktop/f/3515/t/19601491.