Why i can’t upload 15 version on my iphone 7 plus ?

Well, let’s start with the message that is shown in your screenshot. You will need to read and follow that information.

Also: connect to a WiFi network

Also: Charge the battery and preferably leave it connected to the charge circuit during the update process.

Also: Turn off the low power mode option.

There are about a dozen different reasons why the phone is not updating.

To help understand why.......what error message do you see when you try to run an update?

A screenshot would be really great, if possible.

Thanks for the screenshot this really helps.

1) The battery is set for Low Power Mode. The phone will not update in Low Power Mode. Turn off Low Power Mode.

2) The battery is charged to only 25%. The battery needs to be charged to at least 70%. Leave the power adapter connected to the phone during the update.

3) The phone needs to be connected to a Wi-Fi network to run the update. Updates will not run using cellular only.

4) There is not enough free space on your phone to run the update. You will need to delete apps, photos, music, etc to clear more space on the phone. 4 GB of additional free space on the phone will be needed to run the update