How I can download iMovie for free? I have a iPhone 6 128 GB.

This is usually linked to your Apple ID, if your Apple ID has not been used to activate a New iOS device since 1st Stepmber 2013 then that might be the cause.

If you've purchased the device 2nd hand then you won't be eligible for the free apps.

If you did purchase the iPhone new, then try logging out of the iTunes store and back in again by going to Settings > iTunes & App Stores > Apple ID then following the steps linked above.