Why I get notification that my battery is not genuine when I power on my iPhone ??

Because the battery cannot be verified as genuine, which can mean the battery isn’t genuine, or is genuine and there’s an issue with it, usually. If this iPhone has been repaired, check with the repair provider. If there have been no repairs, check with Apple Support. The battery will work as well as it works, though some data is not available from it.

juveria113 wrote:

How should I solve this issue ??

Because the battery cannot be verified as genuine, which can mean the battery isn’t genuine, or is genuine and there’s an issue with it, usually. If this iPhone has been repaired, check with the repair provider. If there have been no repairs, check with Apple Support. The battery will work as well as it works, though some data is not available from it.

Start here: https://support.apple.com/iphone/repair/service/battery-power