Im trying to install macOS on an external drive but it keeps saying that the computer is missing a firmware protection. What the heck even is that?
It looks like your mac needs an EFI Firmware update and somehow your System Software is missing it.
Head to the link below and search "EFI Firmware update" and find the right model of your Mac:
Since 2006, Mac computers with an Intel-based CPU use an Intel firmware based on the Extensible Firmware Interface (EFI) Development Kit (EDK) version 1 or version 2. EDK2-based code conforms to the Unified Extensible Firmware Interface (UEFI) specification. This section refers to the Intel firmware as the UEFI firmware. The UEFI firmware was the first code to execute on the Intel chip.
UEFI firmware security in an Intel-based Mac - Apple Support
Hope this helps and solves your question. Do not hesitate to ask if you need more information.
To make a bootable macOS external drive, that drive must be configured with a GUID partition mapping scheme. An EFI partition is created during that process. The EFI partition holds information specific to that system's hardware.
How exactly are you trying to install macOS on this external drive? What is this drive currently formatted as? Which version of macOS are you trying to install?
Ref: How to Create Bootable USB Drives and SD Cards For Every Operating System - How-To Geek
What is an EFI partition
