First, make sure that the Piece has been marked as sold and assigned to a contact.
You can then generate an invoice in two ways.
The first way is by going to the sale details of the artwork. Click on Artwork in the left-side menu, then click on the Piece for which you'd like to generate an Invoice. In the Sales Info section on the right side, select Sale Details in blue at the bottom of the section.
On the next screen, click on Invoice in the top menu. Fill in the rest of the sale information and click the Generate Invoice button.
The second way to create an Invoice is to click on Income in the left-side menu, then select Sales/Donations underneath.
Click Actions in blue to the far right of the Piece, and select Invoice from the drop-down menu. Select the Piece or Pieces the client has purchased to include on your Invoice, fill in the other information, and click Generate Invoice.
You will be redirected to Reports. Click on the Invoice or the Download arrow to open the Invoice as a PDF.
NOTE: The piece must be marked as Sold to one of your Contacts first. Edit the Sale if you do not see this.