If you are using an “array” as a field name, you retrieve it via the array name, not the array index value…
should not be:
$parcelas = $this->getRequest()->getParam('parcelas');
$parcelas = $this->getRequest()->getParam('payment[parcelas]');
$parcelas = $this->getRequest()->getParam('payment');
$parcelas will then be an array.
You’re code doesn’t really make sense though. A drop down item only returns one result, not multiple results which would necessitate the use of an array (which is a common way to collect values of multiple check boxes).
Your code doesn’t even attempt to handle an array of returned values, which makes your naming the select field with an array structure doubly questionable.