在你申报的第6行 boolean valid = true; 从来没有设定过 false 在代码的后面，所以它不会进入while循环。

根据你的代码，我理解你想选择选项的，如果选项不是select，请在while循环中再次尝试该选项。你可以试试下面的方法。

import java.util.Scanner;
public class CateringBooking {
String serv;
int menuSet;
String timeAndDate;
boolean valid = true;

public String kindOfService() {
    System.out.println("1. TYPE OF SERVICE SELECTION");

    Scanner myService = new Scanner(System.in);
    String selectService = "";
    valid = false;
    while (valid != true) {
        System.out.println("Select the kind of service you want.");
        System.out.println("");
        System.out.println("A: Basic");
        System.out.println("B: Premium");
        System.out.println("C: Deluxe");
        System.out.println("D: Fiesta Deluxe");
        System.out.print("Letter of your choice: ");
        selectService = myService.nextLine();

        if (selectService.equals("A") || selectService.equals("a")) {
            valid = true;
            System.out.println("You've selected: Basic service");
        } else if (selectService.equals("B") || selectService.equals("b")) {
            valid = true;
            System.out.println("You've selected: Premium service");
        } else if (selectService.equals("C") || selectService.equals("c")) {
            valid = true;
            System.out.println("You've selected: Deluxe service");
        } else if (selectService.equals("D") || selectService.equals("d")) {
            valid = true;
            System.out.println("You've selected: Fiesta Deluxe service");
        } else {
            valid = false;
            System.out.println("Your choice is not available. Please try again.");
            System.out.println("");
        }
    }
    serv = selectService;
    return selectService;
}

}