[Plugin: VoucherPress] Sorry, that item was not found

Normally when the voucher can’t be downloaded the reason in in a HTML comment in the code of the page. Just “View source” and you’ll see it.

The link you provide above does not lead to a voucher, so I guess you’ve uninstalled the plugin or disabled it. If you can send a link to a valid voucher I’ll see if there’s a message that will say why the vouchers can’t be viewed.