/usr/bin $ sudo su
sh-3.2# ln -s /Library/Frameworks/Python.framework/Versions/3.7/bin/python3  /usr/bin/python3
ln: /usr/bin/python3: Operation not permitted

sh-3.2# whoami
root

sh-3.2# ls -la | head -n 2
total 139064
drwxr-xr-x  977 root   wheel     31264 24 Jul  2018 .