Windows Could Not Prepare The Computer To Boot Into The Next Phase Of Installation While Installing
I get this message when installing windows with boot camp assistant
I doubt I will be of much assistance since I don't use BootCamp, but you are definitely lacking on details for anyone to assist you. Exact model of the Mac and version of macOS & Windows are definitely needed as well as what instructions you are following to do this (or did you just launch BootCamp Assistant?).
Writing an effective Apple Support Communities question - Apple Community
Maybe this will help (also includes links for later Intel systems):
Install Windows on your older Mac using Boot Camp - Apple Support