New Macbook Pro - remove "high sierra", install "sierra"
Solution 1:
If your MacBook Pro came preinstalled with macOS High Sierra, then you cannot downgrade to older OS as Apple restricted it. You have no other choice. You have to live with it.
Solution 2:
You can't downgrade. Instead, back up all your apps and data and wipe the Mac and migrate things back after Sierra is installed.
You might run into problems if some programs or data upgraded to a new storage format or location.
Worst case, you would need to upgrade again to High Sierra and export any problematic data.