Form 8453-PE is the e-File Declaration form that must be signed. You may have inadvertently uploaded an unsigned copy. Before you e-file your return, I would redo that section and re-upload a signed copy of the Form 8453-PE. I have already e-Filed my return so I am not able to provide the steps for you but maybe in Turbo tax go back to that section by searching for the form number and follow the guided instructions.
One recommendation is when you sign and scan the document with your signature, give the file a name that says mayme "signed-signature" so that you don' confure which file to upload and can select the correct one.
Hope this helps
