This command just put server in warm-up (you can do whatever you want, but you don't get points for that) mode till enough players connect to the server. So if you have a lot of players already on server it'll do nothing.
Btw found a some information about vars.roundLockdownCountdown in server remote administration protocol manual:
Request: vars. roundLockdownCountdown <time: seconds>
Response: OK <time: seconds> - for set operation
Response: OK < time: seconds> - for get operation
Effect: Set the duration of pre-round
Delay: Takes effect next round Comment: Allowed durations are between 15 and 30 seconds for ranked servers, and between 10 and 900 seconds for unranked servers. If the value gets clamped during a set operation, then the clamped value is returned as part of the response
So I believe this is indeed what are you looking for. Try to set it between 15 and 30 seconds. Also try to add it directly into your startup.txt