A B450 board won't boot until is has been updated. When the seller has updated the board, the package will say "Ryzen 3000 ready" but since the launch was three days ago, you won't find any of those for some time.
In order to update the bios you need a 1000 or 2000 ryzen cpu, OR you can do what I did, which is to buy a motherboard than can update the bios via usb, no cpu required. I bought a MSI B450-A Pro, putted the latest bios on a usb, plugged it in, pushed a button on the motherboard and wham-bang-thank-you-mam. I'm gaming with a 3600 no problem.