Embedded systems often use cheap eMMC storage or aging SSDs. These components have a finite lifespan. If the drive has completely died, the BIOS/UEFI cannot see any boot path because the hardware is missing.
The CMOS battery holds BIOS settings. When it dies, the BIOS resets to factory defaults. If the factory default disables your specific boot device (e.g., legacy boot vs. UEFI mismatch), you get this error.
A misconfigured BIOS is the easiest fix.
Before diving into fixes, you must understand the terminology.