Volvo S80 Transmission Service Alert and Gear Shift Issues

I own a 2005 Volvo S80, and recently, I encountered a concerning issue. When shifting the gear from neutral (N) to drive (D), the car suddenly displayed a "Transmission Service Required" alert. Despite the warning, the car seems to drive without major problems, but I am unsure about the root cause or potential risks.

Additionally, I’ve noticed minor thumping sounds and vibrations during gear shifts, which weren’t present before. This has left me worried about the health of my vehicle’s transmission system.

I’m seeking advice on resolving this issue promptly. What might be causing these problems, and how can they be fixed? I would appreciate clear guidance or a recommendation for professional service.