This means that there aren't enough funds in your merchant account to process the refund.
You can activate the Refund Indicator widget on your Home screen to see how much money you have available on your accounts for refunds. For this, in your Live Customer Area go to Home > Customise this page (right top).
From there you can drag the Refund Indicator widget onto your Home screen, which will show the available balance for doing refunds.
Further information to increase the in-process funds can be found in this article.