Isn't the second half of this sentence conditional?

I think you are confusing the double meaning of "could" in the English language. "Could" is either the imperfect of "can" or the conditional of can. In the example above, "...we could not go there..." the word "could" is NOT conditional but the imperfect. Therefore the use of the conditional in French is incorrect. You need the passé composé : nous n'avons pas pu (= we were not able to ...)

-- Chris (not a native speaker).

Bonjour Helen, I think that you may be onto something with your question: here is how I would have stated it --> Nous avions pris nos places à l’avance, mais nous ne pourrions pas aller au final. The thing to keep in mind with this lesson is this: it is about the plus-que-parfait and not le conditionnel. The issue that I have with the phrase is this, I would have used the verb «acheter» --> Nous avions acheté nos places à l'avance . . . the reason being that we had our ticket in advance but were unable to attend. prendre nos places, to me, has the sense that we went to the venue before the show began. Perhaps this is a French locution, prendre nos places, that I have not heard of before. J'espère que ma réponse vous aiderait. Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisé par le monde français depuis l’époque d’Hugues Capet