Photos syncronize with icloud problems

Hi, I am running Mojave from an extern hardrive on a 2013 imac. Most of my pictures are stored in the original hardrive inside the imac. I have not been using Icloud before, and are now setting it up to syncronize and store my pictures. The problem occures when I go to "Photos" and "preferences" and "Icloud", I get the message: "Icloud - functions for Photos are just available for the system library." How do I get by to make icloud sync all the pictures I have on my hardrive? Best Hans