Hi Mary,
You may already know this, but just in case: The Item Charge functionality facilitates recording of fees and charges as a carrying cost of inventory (on the Balance Sheet, as opposed to recording it an expense and flushing it through the P&L). For example, you'd use it to record landed cost instead of expensing freight.
Can you elaborate when you are seeing this error? Are you trying to use it on the Item Journal?
The way you would use this functionality is by adding a line of type "Charge (Item)" on a Purchase Order / Purchase Invoice (or a similar Sales document), assigning a Qty (typically 1), a Cost or Price (depending on type of document), then assigning it by following the steps in the screenshots below (highlight the row, click on Lines in the ribbon, then Line, then 'Item Charge Assignment'):

In the window that opens, you can either manually fill the Qty. to Assign (the total must be 1 for all the rows you have on this screen), or use the Suggest Item Charge Assignment function. In more advanced use cases, you can use Get Receipt Line to record landed cost for freight invoices that arrive separately and cover one or more previously received shipments.

When done, click Close.
Back on the Purchase document, you can fill in the Vendor Invoice number if you have it, and post the document.
I hope this helps. If my response has answered your question, please verify by clicking Yes next to "Did this answer your question?"