Why does the iPhone 6 Plus not have the measurement app after the iOS 12 update but the iPhone 7 does?

The Measure app uses augmented reality (AR) technology to turn your iOS device into a tape measure. You can quickly gauge the size of objects, automatically detect the dimensions of rectangular objects, and save a photo of the measurement for later.

First, make sure that you're using iOS 12 on iPhone SE, iPhone 6s or later, iPad (5th Generation), or iPad Pro. Also make sure that you're in a place with good lighting.

iPhone 5s, 6 and 6 Plus is not supported.

I have no idea. It is called Measure, not measurement. Have you gone into the app store on the 6 Plus and searched for it?