You have to update with an older Ryzen, or contact AMD about a loaner CPU which they have a program to help with. The only other option would be to return the board and get an MSI Tomahawk, Tomahawk Max, or Gaming Pro Carbon. Any of the MSI "Max" boards, are already Ryzen 3000 ready. There may be some other boards that ARE Ryzen 3000 ready, but if so they are not advertising it and it would be pretty hit or miss as far as whether or not you actually get one with a new enough BIOS for it to work.
The Tomahawk and Pro Carbon have BIOS flashback. There are a few other MSI B450 boards with it too. ASUS only has a few of their VERY high end B450 boards, that have it. No others have it.