This could be happening for a few reasons:
Did you select your Choice teams from your computer already? If not go to nba.com/leaguepass/schedule to launch the broadband player on your computer. Sign-in with your log-in credentials and select/save your 5 Choice teams. Next go back to your Roku and exit the NBA Game Time channel and then access it again. You should be able to watch games that your Choice teams are playing in (local blackout rules still apply.)
Are you trying to watch a your local team and are accessing from the local market? If so, the game most likely falls under blackout restrictions if the game is carried by a local TV provider in your area. Example: You live in Atlanta, GA and have the Atlanta Hawks as one of your Choice teams. If you try to watch the Hawks/Heat game from your house in Atlanta, you will not be able to due to local blackout restrictions.
Are you trying to watch a game that does not include any of your choice teams? If so, you will not be able to watch since none of your 5 teams are playing.
Are you trying to watch a live game that is being broadcast on ABC, ESPN, TNT, or NBATV? If so the game is blacked out due to national blackout rules. The replay of the game will be available to watch no sooner than 3 hours after completion of the game.