Have you tried accessing the strike multiple times?
Usually when this happens there is a public event happening near the entrance and it won't spawn you where you're supposed to spawn, and thus you can't gain access. If it didn't work once, try again to see if it works. If you've tried multiple times over an extended period of time and it still won't launch then it may be a bigger issue.
happening to me right now. trying to do the quest step and it keeps spawning me outside the entrance and doesn't let me in. ive tried so many times today for the past hour and still havent spawned correctly