I got the same pop-up message. The glitch is that if your non-cash contributions exceed $500, and you have selected "Appraisal" in the Step-by-step guide, you will need an official appraisal and a physical signature. You also need the appraiser's official number (perhaps a Tax Id number) For reasons unknown, there is no way to get this information into TurboTax (like with an e-signature), so Intuit decided to punt and not support e-file for this special case. Unbelievable.
