For what it's worth, here's my experience.
While waiting for my new Deka battery to come in, I removed the discharged original battery and charged it outside the car using a Schumacher 2A charger/maintainer. It took awhile to fully charge, but it did. The idea was to do this and pop it back into the car to see if it would result in everything working again. The battery is bad and needs to be replaced, but I wanted to see if it was truly the battery. The car's accessory electronics including the ability to open the trunk finally worked again. So the battery was the culprit.
I was too scared of getting stranded knowing the battery wasn't holding a charge, so I didn't drive it. I left it in tact while waiting for my new battery. A couple days later, the 12v battery discharged and went dead again. No power = no startup.
The new battery came in, I installed it per my own video viewtopic.php?f=15&t=17011
and I also got the "unable to charge" notice on my dash even after registering the battery using the BimmerLink app. I've read above and in other sources that people have just left it overnight and it was fine. Since I'm impatient, I decided to take it for a drive to see if that would "help". I drove it around less than a mile, I came back home, turned the car on and off and the error was gone. Check engine light was gone too and the car charges with the blue indicator light on the charge port as normal.
Since your car isn't recognizing power to the car, I'm guessing it has to be one of two things:
1. Poor connection.
Check to make sure the connections on your negative and positive are both snug. Also check that the high voltage connection is snug too. My video in the link above shows exactly how to do it. It's finicky which is why I felt like I need to make that video. There's some BMW documentation on how to do it here (https://www.newtis.info/tisv2/a/en/i01- ... 1VnZ23WOFg
) but no one has put it in video form as far as I know. Hopefully that helps.
2. Faulty battery as noted by others above. You've done the measurement though with your volt meter, so it seems the battery is good. And since you're able to measure it, the metal connectors on the posts that you swapped from the old battery to new seem to be okay too.
Sorry if that's not much help, but definitely let us know when you figure this out. I'm sure it'll help some of us here. Thanks.