Welcome to the documentation:

First, we will talk about what the late bootloader/early kernel says when it thinks your computer is not good enough for the job:

ERROR CODE	|MEANING	
ERR: M		|A problem, e.g you didn't use a multiboot-2 compatable bootloader.
ERR: L		|Your cpu doesn's support 64-bit "Long" mode
ERR: C		|Your cpu doesn't support cpuid, whatever that is.

All these error codes are loaded by loading the letter into the al register, and then jumping to a function called error, where the hard-coded "ERR:" is printed before the error code letter. Download the code for more.