Once I have completed the first situation I can't not get on to any others it's says complete the previous situation and also when I try to get online it says currently not available please check back later does anyone know why this has happened is it because it is a PSN game
Two problems that I see right off the bat.
1. You need an online connection to play most of the modes in R6. Terrorist Hunt (you vs. waves of AI) and Matchmaking (team PvP) both require you to be connected to the internet. You also need a PS+ account to play Matchmaking, Terrorist Hunt's matchmaking, and the secret 11th situation. You can play Terrorist Hunt solo by pressing R3 and selecting LONE WOLF.
2. You have to complete each situation in order. You can't play situation mission 2 until you beat situation mission 1. Is this not happening? That's something you want to talk to Ubisoft with. Ideally you could use the PS4's recording feature to create a short video where you complete the first mission and then try to start the second mission only for the game to prevent you from doing that.
3. Ubisoft has their own support forums http://forums.ubi.com/forumdisplay.php/64-Rainbow-Six