Can't purchase train tickets in advance- not sure what I'm doing wrong

It looks like they are only showing ticket prices a month in advance so you may need to wait awhile. If you wait until you get there, the prices will be higher than if you buy them in advance.
I bought one way tickets in August or September for a trip in October. Just don't put in a return date and after you choose the train you want, it will ask at the bottom "buy only outward". You do need to register with them and they will want to know your passport number.

Hope this helps.

Thank you Leslie! I guess I will just have to wait, lol.

Although that's New Year's Eve, I'd be shocked if tickets sell out by the day of travel; the trains I was on in Portugal in May were not that crowded, and unlike in other European countries, last-minute tickets are not that expensive compared to early purchases. I didn't buy any tickets in advance. It's still only about 30 euros from Lisbon to Porto. (The IC trains are not much slower than the AP but cost more.)

FYI, you have to change at Campanha in Porto for sure to get to Sao Bento. Every ticket to Campanha includes the short connection to Sao Bento.

FYI- On the CP web page half way down, click “relevant information”. There it states the winter schedule will not be available until December 8 and tickets cannot be purchased until then. No, you do not have to purchase round trip tickets, just hit submit after you enter the information for your one way. Yes, you can purchase at full fare once you get to Lisbon.
Happy Travels

"but where I need to go is from Lisboa-Oriente to Porto-Sao Bento."
You do, but the train schedule does not.
Select Lisboa-Oriente to Porto-Campanha.
The 5 minute train ride between Campanha and Sao Bento is free. Some trains from Lisboa actually continue on the Sao Bento, but not all.

The reverse trip has similar quirks. But, not always.
Campanha to Oriente, even if you want to go from Porto - Sao Bento to Lisboa - Santa Apolonia. They never check for tickets between Oriente and Santa Apolonia.

You need to be logged in as a client (it is free to join) on CP.pt to see the promo prices.
My preference is AP 1ª for just a few € more. The promo price range for 31DEC2018 is IC 2ª €9.50 to AP 1ª €26.00 while the list price range is €25-43.

"I'm thinking of traveling to Cascais after Porto" makes no sense to me. Cascais is a short train ride from Lisboa. Tickets are €2.25 and cannot be bought online. You take the train from Porto to Lisboa - Santa Apolonia, then a taxi (€10, because you will have luggage) to Cais do Sodré train station, buy your ticket at the station and go to Cascais. In spite of the fact that Comboios do Portugal runs both trains, they are not really connected in any way.

"Can I just wait to purchase tickets when I fly in?" You can, but that is foolish. The promo prices are so much cheaper.

Give yourself 3+ hours from scheduled landing time to train departure time. The immigration queue is almost an hour. Take a taxi (€10, cash) from the airport to Oriente. You can take the Metro, but you will be very tired. Take the taxi.