When I run the validations on my proposal, I get an error that says "The Attachment [filename] does not exist on the disk at the expected location [location]"
This is caused by a source file located in an attachment point where the document is expecting an actual PDF to be. To fix this error, remove the source files from the locations given in the error, and then upload PDFs in their places.
If this does not resolve your error, or you are unable to perform this, please contact support.