As you know already, you can make refunds either monetarily or in form of a voucher. Sometimes, however, you may be able to do the latter only. Here is a list of possible reasons:
If a booking was paid for with voucher, it can be only be refunded as a voucher.
There are no founds left on your MangoPay escrow account and you have already made cancellations that exceed the value of the security deposit.
You have not uploaded any documents for verification under Account -> Verification.
The following error message appears when monetary refund is not available to you: "Blocked due to a debited User's KYC limitations (maximum debited or credited amount reached)".
To find out why refunding with a gift voucher is usually a great idea, check out the respective Help-Article.