You probably have to go into the bios to get it to boot a USB stick. I just did this, and had to do that. It's a little different nowadays, "UEFI" bios that are very, very biased toward booting Windoze. On our Acer laptop, it reverts to booting the harddrive after every shutdown, so if you want to boot the stick, you have to tweek the bios first. BTW, no manual config of anything needed with ubuntu 18.04. You just make a USB stick using "rufus" and the disk image from ubuntu website, set your bios to boot it, boot it, and "grub" runs, giving you a choice of boot from the stick, or install ubuntu to the harddrive. Everything (like gnome and all that) is totally configured once you finally boot the finished system.