You get this message when trying to set BOOTCAMP as startup in High Sierra:
Apple macOS versions since El Capitan 10.11 have a security feature called System Integrity Protection (also known as SIP or “rootless”). It is enabled by default to prevent third party processes from modifying certain system processes, files and folders.
All system options, including “Startup Disk” are also controlled by SIP. So you won’t be able to switch to Boot Camp environment, if your NTFS volumes are mounted by Microsoft NTFS for Mac by Paragon Software.
Use macOS Startup Disk utility.
Disable NTFS for Mac, then use macOS Startup Disk utility.
Use Boot Menu to set BOOTCAMP as a startup.
Follow steps from this article, but select BOOTCAMP. This will allow you to get around this limitation and boot from BOOTCAMP once.