The Eurail website is updated a bit more frequently than the app but you should not use either to plan because they're not reliable enough.

For Spain, you can plan with the RENFE website or www.raileurope.com. In general, use a national website to plan, or for international planning, the DB website or DB Navigator app is a good choice.

As to reservations: the app cannot make reservations. It only forwards to some of the websites where you can book them. For Spain however, this is of no use, since pass holder reservations for Spanish domestic trains cannot be booked online.

Here are the known ways to book Spanish domestic trains:

In addition, you cannot reserve or book anything in the app. You merely register which train you use, which is the equivalent of, for a paper pass, writing the train in the travel diary. It's also very much recommended not to activate the pass in advance because you never know what will happen. In addition, don't activate travel days in advance either (i.e. using the yellow switch in My Trip). You can only cancel a travel day until midnight before it starts. If you can't travel for some reason, then you may lose a travel day.

You'd better cancel all travel days you may have activated: go to My Pass, select an activated travel day and then tap on Cancel travel day. After cancelling all travel days, the pass will be deactivated. You should only activate it on the first travel day, when you're sure you're going to take the first train.

You also need to answer "Yes" to the question if you want to deactivate your pass when you cancel the last travel day. 

There is a possibility to have an active pass with no travel days, but that you don't want in this case.

Frankly: again someone who is grossly overworried-any person with normal sense takes the bus between these 2 (much more frequent, cheaper as full price, only a little bit slower-and it sets you down fat better to centre town). There are Avant (semi) hi-speed trenes about ev 90/120 mins between the 2 and these are hardly ever full.as read above. If you have a pass with passdays it might not even be worthwhile to use a passday vs normal tickets

This is my first time going to Spain, that’s why I am seeking advice in this forum.

But I have already purchased the Eurail Spain Pass ($800 for 2 adult first-class 8-day pass), is there a way for me to enter my Pass Number on the Renfe site, so I don't have to pay again?

Unfortunately there isn't.

See the link I posted earlier for ways to book reservations for Spanish trains.

Hm….then I don’t understand the point of even buying an Eurail passes?  How do I even use it?  I thought by having the Eurail pass I can save on train fare. 

@thecolorlavender You can't make any reservations for Spanish trains online so you should use the RENFE site. Purchasing tickets on that site is only if you don't have a Eurail or Interrail pass.

You can, depending on what journeys you will make.

But Spanish railways (RENFE) are a pain in the lower backside when it comes to making pass reservations, especially if you want to do that in advance. Fortunately for you, summer will be over soon and from September, it will not be so busy anymore and booking reservations a few days in advance will generally work.

I have the Eurail Spain passes.  Please tell me how I can reserve train rides on Renfe without paying for the train rides again.  I just want to be able to reserve the train rides and seats.  When I selected the train rides on Renfe, it seems I will have to pay for the train rides again, it doesn’t let me redeem my Eurail pass.

I have not been able to find ways to reserve train rides/seats on any sites/apps without have to pay again.

That's because (I repeat) you can't book pass reservations online for Spanish domestic trains. Not anymore, because RENFE took down the interface that e.g. the Eurail website used.

You can book them via other ways. Not all very convenient, but that's RENFE.

There is no way for you to make the seat reservations online for Spain online. 

You need to make the seat reservations at the station in Spain.

Or you can call RENFE  3 days before you come to Spain and make the reservations by phone. The reservations then have to be collected within 72 hours.

How do I use my Eurail Spain Passes then?  I have to wait until I arrive in Spain and then go to a major train station to redeemy my Eurail passes?  Thank you.