The OptiX OSN 6800 is new equipment and several OptiX OSN 6800 NEs have this alarm. It is suspected that the power module is faulty.
After cutting off the power of the OptiX OSN 6800 then power on, the equipment time and date information is lost and changes to the initial time and date, that is 1990-01-01 00:00:00. Then, the battery on the SCC may be faulty.
Procedure

Remove the SCC board and check the jumper that controls the battery. This jumper is found in an incorrect state.
Change the state of the jumper so that the battery works.
Insert the SCC. The POWER_FAIL alarm is cleared.
Reset the date and time.
Reset the equipment and the time and date information is not lost. The problem is solved.