Hi again,
OK, I got it finally fixed! Thank you so much for your help :)
For anyone searching for the help to the same problem, here's how I fixed it:
1. I deleted everything from my Sheet.
2. I wrote the following codes only in the sheet:
Event: Touch -> Is touching left-button Action: Player -> Simulate pressing left
Event: Touch -> Is touching right-button Action: Player -> Simulate pressing right
Event: Touch -> On touched jump-button Action: Player -> Simulate pressing jump
3. I also had to change some Player's movement settings like gravity and jump strength.
Once again a big Thank You for the help and have a nice Christmas!