The reCAPTCHA element is always required, meaning it must be filled out by the user before the form can be submitted
Required error message
reCAPTCHA → Settings → Optional
Change the required error message by entering text into the Error message if required field.
Required indicator text
Settings → Style → Labels
Change the text indicating the element is required. By default this is (required), but you may want to make it an asterisk * for example.