Why doesn't letter [2+i][1] = '+'; work in my method?

I have to do this task for my school where I have to use a method in order to get my Name initials which is M K. I don't know how I can get these initials in this method. The heigt has to be n/2 and the width n/4.Here ist an example picture enter image description here

Here is my current code:

public class Initials {
static Scanner sc = new Scanner (System.in);
  
public static void main(String[] args){
    char [][] feld = createfield ('m','k');
    for (int i = 0; i< feld.length;i++) {
        for (int j =0; j<feld[i].length;j++) {
            System.out.println (feld[i][j]);
          
        } System.out.println();
    }
  
    }
    public static char[][] createfield(char firstname, char lastname ) {
      
        System.out.println ("Parameter for n ");
        int n = sc.nextInt();
        char[][] letter = new char [n][n];
      
        int heigt = n/2 ;
        int width = n/4 ;
      
        for (int i=0;i< letter;i++) {
            for (int j =0;j< letter[i].length;j++) {
              
                letter [i][j] = ' ';
                  
            }
          
        }    for (int i=0;i <heigt;i++) { 
              for (int j = 0;j<width;j++) {
                 letter [2+i][1] = '+'; // For initial M und K
                
              
              
              }
          
        } return letter;
    } 
} ```