Have you tried using a partition manager like gparted to wipe the windows partions?
Another thing is Dell has been doing an awesome (/s) thing lately where they have their disks configured as raid by default on their laptops. Try going into the bios and make sure raid is disabled and the SSD is set to AHCI
Just confirming this being a thing; had to battle the same crap recently at work trying to install Ubuntu server onto Dell Optiplex's... also make sure SATA is set to AHCI and not RAID, as well as the boot options that someone already pointed out in another comment.
So, you've been in the BIOS? I'd imagine it is set to UEFI and you've disables Legacy option ROMS and secure boot?
Somewhere in the BIOS settings there should be a section or entry called Removable Media Boot/USB boot under “device”, “storage” or “boot”. You'll need to toggle an option there.
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0