Translation:The manufacturer could not replace it.
Curious why 'cannot' is unacceptable as this is equivalent to unable, which seems an acceptable definition.
The idea is that "pouvait" is a past tense, hence "was unable", not just "unable". So you may say "was unable" or "could not", the latter being an alternative way of expressing "cannot" in past tense.
It's worth considering how some possibilities are grammatically possible, but a francophone will not necessarily take it that way upon hearing it. Consider the meaning and use the interpretation that is most likely--even if other interpretations are possible. Would a manufacturer replace "him" or "it"?