schenkelberg795 writes, "...we start the GAS tour on Sept. 17 and plan to take the train from Frankfurt to Trier. I’d like to go ahead and book tickets now."
Previously you asked for and received advice on this trip with stopovers in St. Goar and Rüdesheim. Are these stopovers still part of your trip?
If you're still traveling to Trier, whether directly or with stopovers, immediately after your flight arrives at FRA, it's actually better NOT to book in advance. You might see some relatively cheap "saver fares" at the DB site, but these saver fares always include long-distance trains (ICE, IC, etc.) which will lock in your travel to a train-specific schedule. And that could be a big problem. Let's say you pre-purchase a saver fare ticket for the 10:00 am departure from FRA's Regionalbahnhof. You will be booked on 3 trains - one S train to Mainz, one ICE train from Mainz to Koblenz, and one RE train from Koblenz to Trier. That will work fine if you actually catch the 10:00 train. But if you miss that train for any reason (your flight is late, you got confused about where the station is, etc.) you cannot make full use of your ticket because there is no schedule flexibility for the ICE train - and will have to purchase additional tickets.
So the best way IMHO to do this trip is to use the regional trains only. Click on "only local transport" when you do your search for a schedule and you will see only regional trains in the results (RB, RE, S for example.) Any DB ticket you buy for these trains is completely flexible all day long. Also flexible is WHEN you buy the ticket. You can buy it right at the airport when you arrive, and there's no difference in price. The "Day pass for Germany" covers you on these trains and costs €49 / 2 persons - it should appear for all regional train journeys as a ticketing option on the 17th. OR for any journey on the same trains on Friday the 16th starting at 9 am. But there's no sense in buying it online in advance. It is non-refundable, and what if your plans change? Just buy it at the airport once you get there.
There are other ticketing options on the regional trains once you're there as well. I believe Lee already advised you on the RMV + Rheinland-Pfalz ticket option (probably €41/2) which saves some Euros but involves buying 2 separate tickets.
Some regional train departures from FRA involve only ONE change in Koblenz - I would try to get on one of those if possible, depending on what time you reach the airport Regionalbahnhof station.