It would appear that you have defined this new field in Invoice data when it should be defined in Invoices. If you define it in Invoices,you should be able to use this field on both layouts. Be advised that the data entry layout with a portal is not the layout used to print invoices. It's a different layout that is a list view layout based on Invoice Data. To use this field with that layout, you'll need to place it in the header, footer or a grand summary layout part.
Thanks Phil , That sorted it :-)