In 2018 Apple introduced the T2 security chip. The purpose of this chip is to prevent systems other than Mac OS from booting. As such, this chip prevents the FixMeStick and StartMeStick from booting too.
Therefore, we've set the FixMeStick and StartMeStick System Requirements as:
Macs: Intel Based Macs from 2006 to 2017. 2018 and later systems are not compatible.
Note: it is technically possible to configure this T2 Security Chip to boot the FixMeStick, however, the steps are only for the technically adept, and we don't officially support these systems as a consequence.