Лекцию is the accusative case, and it's what we need to use as the direct object of the transitive verb понимать, which, like the majority of verbs, requires that its direct object be in the accusative case. Yes, sometimes negated verbs, especially negated verbs of perception (eg. Видеть) take the genitive, although it's not necessarily required. Putting the direct object of a negated verb in the genitive case was more common in Russian in the 18th century, I believe. Regardless, лекция is the nominative case, which is incorrect here.