Unable to reserver a process on fleet ( FleetCapacityExceededException) – Amazon GameLift

Maybe something misunderstand. Let me explain my game flow:
A client create a room, and wait for others to join to play together.
when client request to create a room by CreateGameSessionAsync method, gamelfit will start 1 game server instance (server build I uploaded ), and if successful, it will return to my client ip address and port of server instance, and client connect to that ip/port to join this room and play. If a game session successful created, it appear on “AvailableGameSessions” metric.
But this exception occur sometimes when client call CreateGameSessionAsync method, mean that can’t create a game session, or gamelift can’t start my game server instance, is that right? I think It’s not related to “AvailableGameSessions” in gamelift console.
In detail of the error, it said:

So I guess it’s something wrong with gamelift fleet?
I think i have intergrated correctly with Gamelift Server SDK, because this exception not occur all the time. Sometimes, i can create game session and join to play normally.