New computer from Dell with pre-installed Ubuntu won't boot. Should I assume it's an error from Dell?

You don't have to accept a computer that doesn't boot from Dell or from anyone. If the computer has a guarantee, return it and make them honor their guarantee. If you do too many things to try to repair this computer, it may be used as an excuse to void the guarantee, and you'll be stuck with a useless unbootable computer.

You could boot off a flash drive and nuke the existing OS installation since it seems to only be a software issue but, as others have said, it is Dell's responsibility and you should file a warranty claim.

In addition to the what everyone else said, there's also the very distinct possibility that there's a hardware problem. Definitely contact Dell.