Trying to boot old computer from usb device, computer wont recognize said device.

Basically i got a super old Dell Dimension E521 (a dinosaur, i know) that im trying to boot puppy linux on via a usb stick. I wrote the iso on the drive to create a bootable flash drive, but in the bios it keeps saying (not present) next to usb device when i moved it up to number one on the boot order. Any recomendations? Tried every usb port on system