There could be a number of things that are causing this error, if you do not have the latest version of pages, you can get it in the Mac App Store. I have provided a link for that below:
***While you are in the App Store (should be in your Applications folder in the dock), you can check to see if there is an update to that program and any other program by clicking "Updates" at the top of the screen.***
Since this is also a known issue on the Mac itself, (removing the older version from the dock by dragging it to the desktop seemed to work in some cases...) double check that the Applications folder does not contain two versions of your program, (one may be in "iWorks" folder) if it does, you do not need to remove the older version, you simply need to make sure you open in the right one.
There are multiple ways to do this. You can drag the file you want to open over the top of the program you want to use and release the button, opening the program and the file.
You can right-click the file you want to open and use "open with..." selecting the program you want to use, opening the file.
You can select the file, go to Apple menu in top left corner of the screen and "get info" (or right-click + get info), and under the " > Open With " click the arrow and select the program to open it with, and now every time you double click it will open that program. (set the newer one of course!)
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