1: Download Launcher
2: Put file somewhere
3: Run MaverickClient.exe
4: Select the Cheat you wish to run
5: Click Launch, Click 'Ok' on the MessageBox
6: CLOSE OUR LAUNCHER, then Launch the Game.
I had the same issues before if I'm not mistaking this message means that you opened GTA after 60 seconds you launch MaverickCheats.
You need to open GTA with in 60 seconds after launching MaverickCheats.