Altium cannot find harness files of design
In Altium 18 and above the Regenerate Harness Definitions has been removed. Further to this, I also think the generation on compile has been removed. The files are generated when a schematic file containing a harness is opened. An easy option is to right click on the containing folder of the schematics in the project window and choose open all. If you open all the schematics at once, all of the harnesses will be generated and your compiler issues should go away