you did not report what model-year Mac you own, and the procedure to allow booting form an external drive varies depending on model-year.
Use an external storage device as a Mac startup disk - Apple Support
Macs from 2017 and older have no restriction on booting from an external drive.
Intel-processor Macs from 2018 to 2020 inclusive have T2 chip, and require the Startup Security Utility to be run from Recovery to allow booting from external drives.
About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support
Apple silicon (all Macs since 2021) also require a similar authorization step.
Change security settings on the startup disk of a Mac with Apple silicon - Apple Support
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