I’ve had this a lot too.Touch doesn’t seem to like some directories. For me it worked if I put the help on the desktop for example, but not in the Touchdesigner folder.
Had the same issue. As long as it is in a location with no spaces in the file name, it seemed to work.
Also it only liked it when I browsed to that location using the offline help dialogue.
When I copy-pasted the file location from windows explorer, it deleted all the back slashes instead of turning them into touch-friendly forward slashes.