Stop start not coming on on my 318 d

https://www.bimmer-service.com/bmw-error-codes/

Question edited for clarity.
Question moved to correct model category.

You need an auto electrician with a pro grade scan tool computer to diagnose the charging circuit and battery drain.

https://www.google.com/search?q=auto+electrician+near+me

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gas or diesel?
that actually makes a huge difference here.

One Drive shaft support And two new flex couplings and you will be good as new!!!!

Did you check your alternator output, Auto Zone will check that out for free.

Try jump starting, see if it starts then. Lights, radio, etc.. don't draw nearly the amps that your starter does. Batteries don't always give warning when then decide to "go south". If you can get it started by jump starting, stop by a local Autozone, Advance, etc.. auto parts store and have them do a charging system test and to load test your battery. They offer this service for FREE
Another Note: When was timing belt last replaced? If it's a 325 of that year it should be done every 60k (If it's 318 ignore the above belt issue)

You have a blown head gasket. It is expensive to repair. I recommend that first you try the products available at Autozone or Advanced Auto. There are several that will clog the blow-out with aluminum powder and stop the coolant from entering the combustion chamber and stop the combustion gasses from entering your coolant system. It is about $15-$20 for these products and I have had success with them several times.

Yes pull the cooling fan relay in the fuse panel. It might be bad.

I cannot see how adding a fuel additive would help to be honest. I would be looking for a split or leaking vacuum line, I have even seen a leaking evap canister before that caused a similar problem. Does the engine pull clean and strong when it is warm or at cruising speeds? Does it feel like its running lean or misfiring?

find the Idle air control valve on the side of the intake and then take off and clean or replace you could try taking a small ball peen hammer and tapping it lightly while it is idling let will genarly free them up