When you chroot'ed at the beginning of chapter 6, the chroot command included a definition for your path:
chroot $LFS /static/bin/env -i \
HOME=/root TERM=$TERM PS1='\u:\w\$ ' \
If you followed the commands to install patch exactly as they were in chapter 5, then the executable should have been placed in either /bin or /usr/bin. So my only guesses are:
1) You accidentally mis-typed the path string when you chroot'ed
2) You accidentally left off the prefix argument for patch
Where is teh static version of patch located on your drive?