You never download a W-2 from the IRS. Your employer sends it to a data warehouse that Turbotax and other tax prep companies subscribe to. You probably have a blank or partial W-2 in your return that can't be substituted with a downloaded one.
If you go back to the beginning of the W-2 interview and click "I have another W-2" it should open the download again, and you download your W-2 as a new one. The delete the blank or partial W-2.
