Sounds like a MBR-GPT conflict, might not be, but that's a great chance. First off. which program did you use create the USB? If it's Rufus, make sure you've got GPT selected-no "MBR" in the option.
Then check your UEFI for a few settings: your B450M's UEFI can be accessed by rapidly hitting the "del" button while it's booting. You'll be met with a red and black screen with options such as "System" or "BIOS" at the top. If you don't see these, hit the F2 button to switch to advanced mode. Then, go to "BIOS" section and turn the setting named "CSM Support" off. Finally, head to "Save&Exit" and hit "Save & Exit Setup" to save your settings. It should reboot and USB should kick in there.
If that does not budge, try forcing the system to boot from it, by using the UEFI's save and exit section, and choosing your USB (The one that starts with "UEFI:") from "Boot Override".