You need to login to Worldpay, find the live account settings for the correct install ID and update a couple of fields.

First, the MD5 Secret. You should copy exactly the same value into the new MD5Secret field you just added into your Kartris back end.

You will also need to add the following to the Signature fields: instId:amount:currency:cartId

Save the settings. If there are errors reported with your MD5 Secret value, fix them, but ensure that you copy any changes back into Kartris, so your MD5 secrets in both places match.