Tasting Menu in Barcelona
My understanding is also that Cinq Sentits only takes reservations 2 weeks in advance, so is it possible they were not booked but that you tried too early? Regardless, though we had a great time there, we liked Alkimia even more, so if you can't get to both you might consider Alkimia instead. Also loved La Dama, which is almost completely opposite in style to the other two, serving superb renditions of classic Catalan cuisine. For us it was a wonderful contrast to the modern style.
There are many excellent places that serve modern Catalan cuisine. My favorite is Sauc: more lively and less formal dining room, the cooking is creative but not too out there; to me, the food has more depth than Cinc Sentits which I like very much. The owner was the chef de cuisine at Abac before opening this restaurant few years ago. More creative but less consistent is Alkimia but my last dinner there was one of the best in Barcelona. Can Gaig is more conservative with excellent service, wonderful overall experience. Less expensive is Coure, the seafood is especially good. Comerc24 can be excellent and is fun and lively, opposite of Cinc Sentits which is very low key. Hope these are helpful.