Lesson 12.7 starts off with the sentence:
Vous ne pouvez pas ouvrir un compte en banque.
Shortly thereafter, we are advised that, "When there is an indefinite article « un, une » ... in a negative construction, the article changes to « de »".
This is clearly not consistent with the earlier example, so I do some digging, and discover that this could be an instance of the Negative Indefinite Adjective, ne ... pas un(e). As I understand it, one has the option, with countable nouns, to use the ne ... pas un(e) negative adjective instead of ne ... pas de for the purpose of stressing the negative. However, this doesn't really seem appropriate for this example sentence (especially w/o explanation). Should this sentence be, Vous ne pouvez pas ouvrir de compte en banque, or am I missing something altogether?
Also, in, tu ne le trouves nulle part, it looks like nulle could also be an instance of a Negative Adjective (ne … nul(le)), as well as, what I believe is, the adverbial phrase, nulle part. I'd like to know what's what here.