Iphone 7 plus when check serial I get this massage (We’re sorry, but this is a serial number for a product that has been replaced.

If you're using the serial number on the box you recieved your iPhone in, the iPhone may have been replaced, as the error message suggests.

You can check the IMEI for your iPhone by checking what's inscribed on the inside of the SIM tray, or you can activate your iPhone and check the serial number by going to Settings > General > About

Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

If it says the serial number was replaced that likely means it was reported as defective and Apple replaced it but they were not returned to Apple as required, or it was reported lost or stolen from stock or during shipping. No support or service is available for it.

I think it extremely unlikely but did you really buy it from Apple, or an Authorized reseller? If you did, click on this link to read how to 'Update your proof of purchase' --> https://support.apple.com/HT202703

If, as I suspect is more likely, you bought it from a third party seller then approach them about the issue. As far as Apple is concerned they will not even look at your device for repair or service. If the device works then I suspect you can use it as normal.

I was assuming you had already checked your serial number:

I buy iphone 7 plus when check serial number