iPhone 6 repeatedly turns on and off

Please try a force reset procedure as shown below, this will determine if this is a software issue.

• Press and quickly release the volume up button.
• Press and quickly release the volume down button.
• Press and hold the side button until you see the Apple logo 

Force restart iPhone - Apple Support

If that doesn’t help rectify your issue, then this is likely a hardware issue in which you need to visit your nearest Apple Store’s Genius Bar or an Apple Authorized Service Provider for service. But also your iPhone 6 is vintage now and ideally any more money put into that iPhone should be put towards a newer model iPhone.

Genius Bar Reservation and Apple Support Options - Apple

Hardware failure. Parts may or may not be available at this point

Make an appointment at the Apple Store or take the phone to an authorized Apple service shop.

You can find a list of authorized service shops in your area using this link: Find Locations

Even if repairs are possible, unless it is something simple like a battery replacement, the money would likely be better spent on a new phone that can run up to date iOS versions.