Where I can reserve just a seat for Barcelona to Lauterbrunnen?

Depends on your choosen connection (which you didn't tell).

If you'd like to go by spanish AVE, you need to do your reservation for this one at a spanish train station. If you'd like to go at 8.19 you'd need a reservation just for this one when connecting to a TER to Genf.

If you'd like to go by french TGV, you may do your reservation with https://www.raileurope.com/pt (desktop version only; add you railpass bottom left before searching). 

For trains in Switzerland itself, a seat reservation is not necessary. 

BTW: August 1st is national holiday day in Switzerland. There will be many events and fireworks; you may have a look for these.

First of all, thank you very much for the tips/information! I really didn't know about the national holiday in Switzerland!!

So I won't be able to buy my seat in advance? This would be the same one you mentioned at 8:19 in the morning. 

My connection would be the one in the attached/below image!

Thanks again!!