If no nominal code has been picked up for an invoice then it usually means there is no line in the nominal grid where the filters match up with the contents of the invoice.
This is a common issue where a new product or outlet has been added to the system but has not been added to the nominal grid.
To resolve this issue you have 2 options.
Add a new line to the nominal grid containing filters that match the contents of the invoice. You can see how to add new lines by checking the "Add a new Line" article
Find an existing line on the nominal grid with the code that the invoice should have picked up. you can then amend the filters on that line to ensure that the invoice will find a match there. You can see how to edit the filters on an existing line by reading the "Edit an existing line" article.