iPhone Back Up to External Hard Drive OSX 10.14 Mojave

Help Please.... Im backing my iPhone X up to my Mac Book Pro using iTunes 12.9. Also Im running Mojave. I watched all the videos on how to get to the "Library" then "Application Support" and then to "Mobile Sync." I then copied the "Backup" folder to my External Hard Drive. I then Erased the "Backup" folder from the Mobile Sync folder. After that I was told to paste something like this:

ln -s /Volumes/mac1tb\ 1/Backup /Users/djtee1tb/Library/Application\ Support/MobileSync/Backup

It was created with my external direction as well as my internal HD name. after 100 different variations and toys pasting it into TERMINAL I get this:

"Operation Not Permitted"

I see older posts where this worked. Is it not working for me because I updated to Mojave? Am I doing something wrong? Im confused. Anyone have any idea how to accomplish this task using Mojave? Thanks in advance.