I believe your while condition is being met by the invalid input.
Perhaps try something like:

String userInput = "";

while (true) {  // loop until valid input 
  userInput = scanner.nextLine();
  if (isInteger(userInput)) 
    break;
  System.out.print("Please enter a valid number.");
}
  
private static boolean isInteger(String str) {
  try {
    Integer.parseInt(str); 
    return true;
  } catch(NumberFormatException e) {
    return false;
  }

These previous answers may also help: https://stackoverflow.com/a/53904761/6423542 https://stackoverflow.com/a/24836513/6423542