Изменить стандартное сообщение об ошибке в input

function replaceValidationUI(form) {
  // Suppress the default bubbles
  form.addEventListener("invalid", function(event) {
    event.preventDefault();
  }, true);
 
  // Support Safari, iOS Safari, and the Android browser—each of which do not prevent
  // form submissions by default
  form.addEventListener("submit", function(event) {
    if (!this.checkValidity()) {
      event.preventDefault();
    }
  });
 
  var submitButton = form.querySelector("button:not([type=button]), input[type=submit]");
  submitButton.addEventListener("click", function(event) {
    var invalidFields = form.querySelectorAll(":invalid"),
      errorMessages = form.querySelectorAll(".error-message"),
      parent;
 
    // Remove any existing messages
    for (var i = 0; i < errorMessages.length; i++) {
      errorMessages[i].parentNode.removeChild(errorMessages[i]);
    }
 
    for (var i = 0; i < invalidFields.length; i++) {
      parent = invalidFields[i].parentNode;
      parent.insertAdjacentHTML("beforeend", "<div class='error-message'>" +
        invalidFields[i].validationMessage +
        "</div>");
    }
 
    // If there are errors, give focus to the first invalid field
    if (invalidFields.length > 0) {
      invalidFields[0].focus();
    }
  });
}
 
// Replace the validation UI for all forms
var forms = document.querySelectorAll("form");
for (var i = 0; i < forms.length; i++) {
  replaceValidationUI(forms[i]);
}

for (var i = 0; i < invalidFields.length; i++) {
      parent = invalidFields[i].parentNode;
      parent.insertAdjacentHTML("beforeend", "<div class='error-message'>" +
        invalidFields[i].setCustomValidity('Plz enter on Alphabets')+
        "</div>");
    }

for (var i = 0; i < invalidFields.length; i++) {
  invalidFields[i].setCustomValidity('Plz enter on Alphabets');
  parent = invalidFields[i].parentNode;
  parent.insertAdjacentHTML("beforeend", "<div class='error-message'>" +
    invalidFields[i].validationMessage +
    "</div>");
}