I had this error as well using a downloaded version of Turbotax business. It also didn't show up under the Tools as a valid attachment. I moved the file from My downloads folder and put it in the same folder where I was saving my return. I also changed the name to something simpler like "signature form". The pdf previously had a name with unusual characters. I was able to check that it attached by going to the tools menu. Then I was able to efile.
