Downgrade macOS Sierra of a brand new MacBook
Macbook Pro 2015 came with El Captain version pre-installed in factory therefor you can easily revert back to Factory installed OS by using Internet Recovery.
Don't forget to Backup everything. (since it is a new Macbook pro, maybe you don't have much in it yet to backup? otherwise backup everything. Use TimeMachine to backup)

Now, turn off your Macbook. Turn it on and hold Command+Option+R
You will see a rotating earth icon. Connect to your WiFi and let it download and install El Captain from Apple servers.