Hello Bruce,
Most Spanish train tickets are only available about 60-days ahead, so it's probably to early to book tickets for the end of Sept. As you have already found out, Renfe is not the most user-friendly site. I prefer to use Trainline, which is much easier to use. Looking at Trainline for trains on this route on Sept 25th, I do see several trains available, but there is also a message saying "Some train times may be missing due to a problem at Renfe's end. If you can't find the journey you're looking for, please try again later." As stated above, my guess is because it's too early. You can do a search for the same day of the week (Friday), about a month from now, to get an idea of the schedules and ticket prices.
As mentioned by WengenK, there are cheaper, advance purchase tickets available the sooner you book, which are also more restrictive, so be sure to read the fine print and be ready to commit at the time of purchase.