I had the same issue and just fixed it quite literally now. (At least it worked for me.)
In the Games tab within Vortex, you click the three dots in the corner of the Fable Anniversary game rectangle and then (provided it has the correct directory path there) you click the Manually Set Location which opens up a file browser window (Likely at the actual correct path of the game.) and you simply don't move anywhere and click Select Folder right there.
After that, you should be able to activate the game. (At least I was.)
Best of luck mate.
/Your friendly neighborhood Christmas miracle I guess.
