Q: car won't start even after taking it to the dealer

Did you mean to say "the alternator was charging as per the test" when you were relating the dealers comments? It is actually the alternator that charges the battery. I suspect the battery and alternator are really fine - and perhaps the dealer thought that’s what you were checking on. The problem is that something is draining your battery at night after you shut the car off (OnStar, for example, has been know to stay "active" after the car has been shut off, causing dead batteries).

Testing the alternator for proper charging doesn’t tell you that something - and what, is draining the battery every 3-4 days. It just confirms that the alternator is trying to recharge the battery when it has a low charge. It would appear, based on age and low mileage, that your Verano is still under factory warranty. I would recommend you take the car back to the dealer under warranty and insist they determine what is draining the battery while it’s just sitting still. Escalate to Buick Customer Service if needed.

If you are not getting an adequate response from your dealer, or the problem is not covered under warranty, I would recommend a battery is not holding a charge inspection to determine what is causing the drain on your battery repeatedly.