Just called TK 5 times, and finally got issued a ticket from SEA-IAH-SJU

UA seems like update new X class ticket at 0:00 PST. I called TK straightaway when I see UA open a X class ticket. Different agents from 01:00-03:00 PST kept on telling me that the flights are not opened to book, despite the system letting me to book SEA-IAH, and IAH-SJU separately.

Finally booked a flight at 03:00 PST when an agent helped me successfully book this flight after more than 4 attempts