You have encountered an issue where the custom routing cost centre assigned to this supplier cannot be updated. This is because there is no cost centre code present in the custom routing rule applied to this document.

To resolve this issue, you need to add a cost centre code to your custom routing rule. Here are the steps to do so:

1. From the alerts screen, go to the "Cost Centre" column and open the drop-down menu.
2. Search for and select the correct cost centre code from the options available.
3. After selecting the correct code, click the + button on the right of the document to save the updated rule.

By following these steps, you will be able to ensure that the custom routing cost centre code for this supplier is properly updated and that your documents will be routed correctly.