Do you have created scan exclusions in your security software as described in the trainer's instructions and launched the trainer with admin rights?
Make sure to do this first
THEN, make sure you are using the trainer properly. Run the trainer BEFORE the game. When you run the trainer you should press F2 as the instructions say, then select the root Far Cry 5 directory. The trainer will run the game for you.
The reason you are getting this error is because the game still has anti cheats blocking the trainer and the trainer didn't run the game.
F2 is THE only way you could use our trainer from the moment we put it out. It disables the anticheat.
The option to save Custom Settings is for saving customized key assignments, not modified values of certain options.