If you are using the front USB ports try the lower ports.
Make sure that you have formatted your live USB properly.
On boot press F12 to get the one time boot menu, if everything is right you should get a menu that gives you legacy boot options with UEFI boot options below that.
The big thing here is that not all live distros appear to work with the Dell UEFI implementation (got Linux Lite and Manjaro working)