Message telling me battery is not a genuine apple product?

phone keeps turning on and off with the message I recieve of maybe not a genuine apple battery

This message can appear for several reasons:

1) The battery was replaced with a non-Apple part

2) The phone is detecting a possible problem with the battery or power supply....even if the battery is genuine

Perform a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation.....or, if it doesn't.....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations

Ok, a couple of things to check:

• What is the battery health percentage?
• Does the battery health page indicate that it needs serviced?

Check the iPhone battery health and usage - Apple Support

• Try a Force Restart

Force restart iPhone - Apple Support

• Is there a software update available?
• Which iPhone do you have?

You need to provide the obvious detail. Did you recently have your iPhone serviced?