You have one or more items on the invoice that have a style code of U on the item, and you don't have a detail section of type U on your invoice specification. It could be that the item was edited to change the style code to A after it was added to the invoice. It could be that style U is assigned to Inventory items (in the Company profile).
To see what Style codes are saved on your invoice, edit your invoice and on the tab that lists the items, use the column editor to add the 'D - Spec code' field as a column. I bet you will find one or more with 'U' instead of 'A'.
If you really want style A for all your items, you have to delete and re-insert the item onto the invoice. Or if there are a lot of invoices that have the problem, Archive the batch, run the Update Archive Batch function for items, then restore the batch to a new Open batch.
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Regards,
Softrak Tech Support