Digital Signature on File Won't Save

Hello all. I am putting together a form in excel that has a required signature line. I'm going through the Insert >> Text >> Signature Line to place the box in the document. Everytime I reopen the file and try to get someone to sign and send it back, it keeps telling me that the signature is invalid. Searched this out on Google, and cannot find a solution. Despite following instructions found to a T, it's still not working correctly.

Does ANYONE have any idea how to fix this?? I'm at my wit's end with this file.