Here you can read the answer to your question. 

https://community.eurail.com/train-connections-reservations-47/trying-to-reserve-a-required-seat-from-madrid-to-barcelona-but-only-option-is-11916

But I still do not understand. I am now in the Netherlands. Is there a way for me to reserve a ticket for barcelona to madrid on the 4th of august and back on the 7th? I do not want to miss the seat

There's a list of known options here:

How long in advance are you in Spain? It's usually no problem to get a seat reservation a few days in advance on such a busy route. Can be booked anywhere in Spain.

I Will arrive in spain on the 28th of july. Is there no earlier option to book or call someone? 

Please have a look at the post I mentioned above.

Yeah I saw that post. But for me it is not totally clear that if I do this 72 hours before I have a ticket for sure. Because I need to be on that train that day to madrid and i can not being denied. I Just want to be sure I can have a place. So you tell me you need to do this 72 hours before in spain? And you have a place for sure?

No one can tell that you will have a place for sure if you made a reservation by phone.

You should check if there is a minimum time before departure of the train that the ticket has to be picked up if you make a reservationby phone. I read somewhere that the ticket has to be picked up no later than 24 hours before departure, but I have not been able to confirm that.

I suspect the only way to guarantee that seat would be to buy an advance ticket from the Renfe site or a site such as trainline - currently 56 euro.

 

Thank you. So could I call now already to the spanish train station to reserve a ticket for barcelona to madrid or not for the 4th of august?  Because 56 euroes is a bit too much if I pay 12 with interraill.. I Just want to know how it works for that train connection, that is why I asked this. 

Once you call, you must pick up the ticket within 72 hours so unless you are going to Spain within 3 days, calling now is not possible. 

But lets say I call on ghe 2nd of august then. Is there a possibility there is no seat left anymore for the 4th?

On Barcelona-Madrid there are lots of trains so the chance they sell out is quite small. There are no pass holder quota on Spanish domestic trains, so you can book until the train is full.

But if you arrive in Spain already on the 27 July, just go to a ticket office and book all your Spanish trains there. Or call 72 hour max. hours before you can go to a ticket office.

Or you email the travel agency recommend in the link I posted as the first reply to your question. 

You said earlier you will be in Spain on 28 July. You can on that day go to a ticket office in any main Spanish station and get your reservations from them. There is no time limit on in-person reservations at stations.

Or by phone call them  2-3 days before the 28th to pre-reserve. the pre-resrvation will only be held for 72 hours for you to pick-up and pay at a station ticket office, once 72 hours passes the pre-reservation hold is automatically cancelled, that is why there is a 72 hour limit on phone reservations.

Unfortunately nobody here can guarantee that there will be availability on any given train at any given time however experience tells us that it is very unlikely for there to be no availability Barcelona - Madrid even at short notice.

For example right now at 16.17 there is space on departures today at 17.00, 17.25, 18.00, 18.40, 20.00 and 21.00 as well as 16 trains tomorrow from Barcelona - Madrid.