We invite you to check first that the credit voucher is linked to an order on the same site as the order you wish to place (France, Uk, Us, Global, Germany, etc..).
The credit voucher can only be used on the site for which it was offered, and no changes will be possible.
Credit vouchers are not cumulative.
Also check that you are using the email address associated with the credit vouchers, that it is still valid and that you are on the right site.
If the expiry date has passed, it will not be renewed.