See https://documentation.unbounce.com/hc/en-us/articles/360001198903-Modifying-Form-Validation-Error-Messages

Customize form validation errors.html

<script>
  
  // Change the message when required field is empty. Replace `your_name` with your form field ID.
  window.ub.form.validationMessages.your_name.required = 'You forgot to enter your name!';
  
  // Change the message when email address field is invalid. Replace `your_email_address` with your form field ID.
  window.ub.form.validationMessages.your_email_address.email = "That doesn't seem to be a valid email!";
  
  // Change the message when phone number field is invalid. Replace `your_phone_number` with your form field ID.
  window.ub.form.validationMessages.your_phone_number.phone = "That doesn't seem to be a valid phone number!";
  
</script>