I was able to generate single-spec BUPs for devices I have access to when they are connected in recovery mode, but I no longer have direct access to some of the devices with a different TNSPEC.
you may running script file, build_l4t_bup.sh to generate BUP.
please see-also developer guide, Bootloader Update Payload Generator.
there’re two sections for generating a single-spec BUP, or a multi-spec BUP.
please refer to sample commands for reference, thanks