Pages is not going to open that document for some reason. Hopefully, you have been using Time Machine from before Pages failed to open that document. If you do not perform hourly Time Machine backups to an external drive, you can ignore the rest of this post.
Rename the problematic Pages document. From the menu extras panel, click on the Spotlight 🔍 icon and enter the following in its panel:
machine
You should have two entries for Time Machine, and click the one associated with Applications. This will launch Time Machine and a series of Finder windows will stack on screen. Locate the Pages document on the Desktop of these windows, and then go back in time to before you could not open the Pages document. Select it and click Restore. That document will be saved to your Desktop, where you can reattempt an effort to open it in Pages.