Why can't newer Mac hardware boot DBAN?
T2 chip in newer Macs needs to recognize the boot media to allow it to start from that disk. You might be able to tell the system to use the drive as a startup volume if you can log in first with sufficient privileges (Preferences --> Startup Disk). Workaround? Boot to recovery partition, open Disk Utility, and do a secure wipe of the drive, using highest stringency options. Accomplishes the same thing (7 pass U.S. Department of Defense 5220-22-M compliant wipe) without the need to run DBAN. Also leaves the machine in a recoverable state so that the next user can install from the recovery partition, which is far faster than Internet Recovery (and in some cases, DBAN wipes the recovery partition, which makes reinstalling incredibly painful depending on the machine - I spent several days repairing an old Air after our IT folks used a DBAN equivalent to wipe it - even worse, this happened AFTER I had already run a full wipe from disk utility - the DBAN wipe wasn't even needed at that point!)