you are either
not running the right trainer with the correct version of the game (uplay vs steam)
not running legit steam version
have some sort of bizarre install where your installation is not the same place as the uninstaller is saying it is
there is no way for us to know. the trainer is looking for the registry location that steam stores the uninstall info for the game as to where the game is located.
the trainer is not sending windows the correct location due to any number of things that I will probably never be able to help you find.
things to try
make sure you have steam version of trainer
use uninstaller to uninstall the game
make sure you own legit steam version
I have no other suggestions, and than perhaps your version of windows or your localiziation is storing registry items in a different location than mine (i.e. windows 10)