I have Mavic 2 pro which has 3 batteries.
One of the batteries will not let the drone fly, in the app it says

"
Battery Check
It is detected that the battery maybe not be inserted.
Please confirm the flight safety before taking off.
"

The battery clicks in just like the other 2, a good secure fit no looseness no visual difference. The connector looks fine. It will power the drone up but will not let it fly. I can still charge the battery with no issues. Its just when i go to fly that message appears on the app and prevents it from flying.

Hi, PC2CV.  Thank you for reaching out and we're sorry for the incident that happened with your battery. Please try the following steps below:

1. Check if the battery buckle is properly mounted.
2. If the battery has already been properly mounted multiple times but the DJI Fly app still prompts "Battery installation error. Aircraft unable to take off. Ensure battery is installed properly", in this kind of situation, I'm afraid that you may consider sending the unit in for a proper diagnosis.  You may submit an online repair request through this link: {https://repair.dji.com/repair/index}. If you need assistance with creating a ticket, you may contact DJI support at https://www.dji.com/support. Your kind understanding is appreciated and thank you for your continued support.