Actually I don't think gigabyte's 450m ds3h supports flashing the bios without a working cpu first, if that's the case (assuming it is the bios) then you would either need an older ryzen cpu to boot and install the new bios or you can request a free boot kit from amd
here
If you are able to find out what version BIOS your current B450m has you can see if that version is compatible with the 5600x
here
If there's any other suggestions that come up I would try those as well since I know getting a boot kit can take time
When did they buy this motherboard if I might ask, and was it used?
Because it would seem odd if gigabyte hasn't been updating the BIOs on their boards for the 5000 series ryzens which came out in october 2020