You dont need to "schedule service just in case". You need to go directly to the service center, like today, if its someplace you can get to fairly easily. If its not, you need to schedule service, then call the local service center and ask for expedited handling / mobile service for your 12v battery replacement.
You are lucky to be receiving an alert for it, but if you dont try to handle this NOW (not "tomorrow" or "next week when the appointment comes", it is EXTREMELY likely that in the next 1-5 days you will go out to the car, it wont start, and you will then be getting it towed to the service center.
Here is one such thread:
Today my wife's Model 3 displayed a dialog saying the 12V battery has degraded and should be serviced. The car is 3 years old, so I know it's not unusual to have to replace the battery at this point, but I was wondering how urgent the replacement is. There have been no symptoms of 12V battery...
teslamotorsclub.com
Notice on Dec 4th regarding 12v (low voltage battery). I posted this same warning to them on that day (dec 4th)
Next day (dec 5th) car dead in garage: