Welcome to FT!, phxrsng
Three things required
1) Inventory in a fare class exists
2) A fare rule for that fare class exists for you routing
3) the itin meets all the fare rules -- min stay, advance purchase days, routing, day of the week, ....

What is the number in your S fare -- 14, 21, .... that usually means the number of days prior to travel the fare can be purchased.
Only way to get around this is wait until SDC which waives the advance purchase requirement.