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

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

1. From the alerts screen, go to the "NL Extension" column and open the drop-down menu.
2. Search for and select the correct Nominal Ledger 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 Nominal Ledger code for this supplier is properly updated and that your documents will be routed correctly.