2K Sports has announced that it has shut down the online servers for NBA 2K16 already. This means you will have to buy NBA 2K17 or NBA 2K18 to play online.
The games that are now functional online include NBA 2K17, NBA 2K18, WWE 2K17, WWE 2K18, Top Spin 3 and Top Spin 4. Any other 2K Sports game is offline forever now.
Since NBA 2K is an annual franchise, servers are only kept up for around two years or so. It’s to mainly force players to migrate to play/buy the newer games each and every year.