If you accepted a voucher without the air carrier informing you of your rights for a refund, you are eligible for a conversion of the voucher to a refund. In that case you should contact the air carrier in order to ask for a conversion.
If the company informed you of your rights for a refund, but you choose the voucher the air carrier is not obliged to convert your voucher.
If the air carrier doesn’t want to convert your voucher you can contact the authority from the country the flight was to depart from.