Does this mb have the asus ez flash thing? Definitely Check the manual if you havent.
If not, i remember AMD having a "loaner processor" program where they ship you out an old outdated cpu so you can use that to flash the bios, and then you return it. I don't know if AMD still offers this.
If the motherboard doesn't recognize your Ryzen 3400g that means you don't have access to the motherboard's BIOS so you can't update the BIOS using Asus BIOS updating software since you need to enter BIOS to run them and your motherboard doesn't have the BIOS FLASHBACK feature which can update the Motherboard's BIOS without a CPU installed.
See if you can borrow a CPU that will work with BIOS version 805 or older and install it to update the BIOS or take it to Best Buy and have them update but it will probably cost you around $100.00 or a computer shop that might charge you less.
Not sure if Asus Warranty will update the BIOS for you if your Motherboard is still under Warranty.
Otherwise you will need to get a AMD BOOTKIT to update your BIOS version to the correct version 2007. You can start by opening a AMD Warranty Request and asking for a AMD BOOTKIT from here: https://www.amd.com/en/support/kb/warranty-information/rma-form
From AMD Support concerning the AMD Boot Kit: https://www.amd.com/en/support/kb/faq/pa-100
Even though it says it is for the 400 Series Motherboards but now I believe it also includes the 500 Series Motherboards as well. This doesn't look like it has been updated for the 500 Series Motherboards.