This error message seems to be the last ditch answer from the App Store when it does not recognize something that is too new for it.
We could not complete your update.
The product distribution file could not be verified. It may be damaged or was not signed.
For example, on Yosemite, I cannot open .pages files sent to me from users who are already on El Cap. When I try to upgrade Pages, as it suggests, I get this error. The only way around it is to upgrade to El Cap so I can upgrade Pages. Not ready to go there quite yet, so I'll just ask for the files in some other format for now.
![User uploaded file](https://discussions.apple.com/content/attachment/644465040)