The company would demand a payment since she (missed)/(had missed) the last.
'had missed' is correct here because it happened during some time in the past before scecific event(deadline of period when it's allowed to pay).
But is 'missed' ok to use here? This is like single event in the past.
If needed, there's full sentence:
Out of that there was food to buy for the two of them, Estella's school fees, plus rent of the tiny flat they lived in; also the finance company would demand a payment since she (missed)/(had missed) the last.