Fare Classes

This is going to sound silly but in the recent forms I couldn't find anything on this.

I'm trying to book a flight for Friday and under fare class available it has S 4 and T 2.

Now I know that T is lower than S but it's only letting me purchase an S fare. How do I get the lower T fare? Do I have to wait for them to release it?

Do I call instead of using united.com?

Any explanation would be great!