No matter how simple your form is, users will make mistakes like not filling in all the obligatory fields or inserting text instead of numbers in the phone number field. This will prompt an error message to appear. To change the default text of the error message for the form (or shopping cart), go to Site settings → Forms → General forms settings and add the text.
Note. A customizable error text for the form (or shopping cart) is not available for the Free plan subscribers.