problem for downloading dtb to self-designed xavier main board

Are you using custom board? As there is error info “Unsupported boot-device strap-reg: 0x00000004” which might suggest the boot device issue (eMMC is not chose as boot device?), you should check the strapping pins status on carrier board. Please refer to the strapping pins chapter in OEM DG to find out which pin’s status might be wrong.