pages won't open template

Log into another user account on your Mac and see if the problem persists there. That will tell us if the problem is limited to your user account or is system wide.

If it persists in the other account boot into Safe Mode, Use safe mode to isolate issues with your Mac, and try from there.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some cache cleaning, volume verifying and directory repairing.

If you are attempting to use Pages '09 v4.n (purple ink bottle icon), then it has been rendered useless by Mojave, and you can longer open existing, or create new documents via any template choice. Use Pages v7.3 in the Mac App Store, or look for a better application.

Verify your software is up to date.

>System Preferences>Software Update.

What's new in Pages for Mac - Apple Support

Pages on the Mac App Store - iTunes - Apple