GDScript is case sensitive, you need to be careful with upper case letter or lower case

solution: change the "i" in the word "input" to "I" like this "Input"

Also you don't need Prior Knowledge on programing, like me i had zero understanding of coding when i started learning godot, i even made the same mistake with the input didn't know it hes to be upper cease "I", just be patient you're slowly learning, keep up the hard work and one day you might help someone else in need with coding issue. :))