This is common with apps not downloaded from the App Store. To bypass:

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Search for "iMovie 10.2.5" to see if a user has uploaded a backup of the original DMG or application bundle. MacUpdate / Softpedia:

A: Yes. If you have a Mac with iMovie 10.2.5 installed, copy the iMovie.app from the Applications folder to an external drive, then drag it into the Applications folder on your older Mac. No DMG required.

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