Hi,
From looking at the manual for your motherboard, I can't see any reference to being able to do a bios update without a CPU, so you'll need a compatible CPU for the BIOS the board is using.
From the ASUS support site, support for 5000 series CPUs was added in bios versions 3402. The following was taken from the ASUS support bios section.
PRIME B550M-K BIOS 3402
Version 3402
15.83 MB
2023/11/06
"1. Update AGESA version to ComboV2PI 1.2.0.B
2. Update AMD 5000/3000 Series CPU fTPM version, please back up Bitlocker recovery key before updating this version BIOS"
If your system won't post with the 5600 then you'll need to find an older Ryzen CPU, install that and then update the bios to the latest, then install your 5600. If you don't have an older Ryzen CPU then you can try seeing if a PC shop you bought the m/b or CPU from can do the bios update for you. Some may do it for free, some may charge a nominal fee.
Best of luck
Craig