Click on Artwork and then Pieces on the left-side menu. Select the Artwork for which you want to generate a Certificate of Authenticity. Click the name of that artwork.
Click the gray New Reports button and select Certificate of Authenticity. Select the information you want to include under Options and click Generate.
You will be redirected to Reports. Click on Certificate of Authenticity or the Download arrow to open a PDF.
Example Certificate of Authenticity: