Why do I keep getting this error " Incompatible operand types char and String" [closed]

I just don't understand why I keep getting this error! Incompatible operand types char and String

Here's the problem... (Phone keypads) The international standard letter/number mapping found on the Write a program that prompts the user to enter a lowercase or uppercase letter and displays its corresponding number. For a nonletter input, display invalid input.

public class PhoneKey {

public static void main(String[] args) {
    // Create a Scanner //

    Scanner input = new Scanner(System.in);

    // Declear variables //

    char letter;

    // Take input from the user //

    System.out.println("Enter a letter: ");
    letter = input.next().charAt(0);

    if (letter == 'A' || letter == 'a'|| letter == 'B'|| letter == 'b' || letter == 'C' || letter == 'c'){
        System.out.println("The corresponding number is 2");
    }
    else if (letter == 'D' || letter == 'd'|| letter == 'E'|| letter == 'e' || letter == 'F' || letter == 'f'){
        System.out.println("The corresponding number is 3");
    }
    else if (letter == 'G' || letter == 'g'|| letter == 'H'|| letter == 'h' || letter == 'I' || letter == 'i'){
        System.out.println("The corresponding number is 4");
    }
    else if (letter == 'J' || letter == 'j'|| letter == 'K'|| letter == 'K' || letter == 'L' || letter == 'l'){
        System.out.println("The corresponding number is 5");
    }
    else if (letter == 'M' || letter == 'm'|| letter == 'N'|| letter == 'n' || letter == 'O' || letter == 'o'){
        System.out.println("The corresponding number is 6");

    }
    else if (letter == 'P' || letter == 'p'|| letter == 'Q'|| letter == 'q' || letter == 'R' || letter == 'r' || letter == "S" || letter =="s" ){
        System.out.println("The corresponding number is 7");
    }
    else if (letter == 'T' || letter == 't'|| letter == 'U'|| letter == 'u' || letter == 'V' || letter == 'v'){
        System.out.println("The corresponding number is 8");

    }
    else if (letter == 'W' || letter == 'w'|| letter == 'X'|| letter == 'x' || letter == 'Y' || letter == 'y' || letter == 'Z' || letter == 'z'){
        System.out.println("The corresponding number is 9");
    }
    else{
        System.out.println(letter +" is an invalid input");
    }


}

}