Downgrading from Mojave to Sierra will require that you erase and reformat the drive. ALL data will be erased.
Make sure you have a backup copy of all of your documents and data before you attempt to reinstall that earlier OS.
The easiest way to install the earlier OS on the new MacBook Pro will be to make a Time Machine backup copy of the old machine, and then use the backup to restore that OS onto the new MacBook Pro.
Please see: Back up your Mac with Time Machine - Apple Support
Also see: Restore your Mac from a backup - Apple Support
OTHERWISE, you'll have to create a bootable installer for 10.11 El Capitan, use that to install El Capitan, then upgrade to 10.12 Sierra after downloading the installer disk image for Sierra. That process is as below.
1) Download Mac OS X 10.11 El Capitan and create a bootable USB installer. Use that to load El Capitan on your computer.
Please see the instructions here: Create a bootable installer for macOS - Apple Support
2) Download the disk image for the macOS 10.12 Sierra installer, expand that and launch the installer.
Follow the instructions here: How to download macOS - Apple Support