OS is up to date, and so is Pages.
Quit Pages, and reboot your Mac. Launch Pages again with the shift key pressed, then try to open this file again. Usually when you see this message, it is a grim prognosis for opening it again.
Try opening in iCloud.com.
+ If it doesn't open, the file is corrupted--you'll want to pull it from a Time Machine backup.
+ If it does open, make a minute change to the document, let iCloud save it, open it again in Pages (assuming you're using iCloud Drive).
Check the file size. If it is next to nothing or way small, it is corrupted.
Peter
I can't open a Pages document created with Pages 5.6.2
