Canon 6D - мигают буквы FALL, либо FA11. Что это? И что с этим делать?

Тебе правильно написали - изучай инструкцию. Функционал камеры нужно изучить системно а не урывками, только тогда тебя не будет ставить в тупик каждая мелочь.

Если подробно, то:
Canon AUS was not able to advise what caused this 'error' message, except to say that they were sure that it wasn't an error and that the camera was fine to use and should operate normally. They contacted Canon Japan for more information, which took a few days, but they got the answer. When the techs adjusted the lenses to the camera, the camera is plugged into their computer/system, and needs to switched into some special mode that allows the AF adjustments to be made. When the adjustments have been completed, the camera should be switched back out of that adjustment mode by flicking a switch or ticking a box in the computer software (not in the camera). It seems that step was not performed when the adjustments were completed. The message probably means "FAII" for Focus Adjustment II, or something like that.
Короче, накосячили при юстировке, несите обратно, все исправят.

Вот сайт.
canon.ru/support/consumer_products/products/cameras/digital_slr/eos_6d.html?type=errorcode

Вводите в поле номер ошибки, который вы не написали тут, если хотите. Всё равно потом понесёте в ремонт.

несите в асц https://www.canon.ru/support/consumer_products/repair/