All of the newer Mac laptops come with a T2 chip. If you are wondering what a T2 chip is, here is a typical definition.
Newer Mac models with Touch ID, Touch Bar, and/or the T2 security chip default to having a secure boot mode that disallows booting the Mac from external startup drives. This security setting is recommended for most users to keep enabled
The T2 chip is all about security. Part of that security is it will not allow a Mac to boot from an external drive. If you wanted to boot from an external drive to do a clean OS installation or for some other reason, you need to change some settings in your operating system. This article explains how to do that.