You can change / update validation and error messages via a filter available in PowerPack’s Subscribe Form module i.e. pp_subscribe_form_strings_i18n introduced in v2.12.0

Please add the following code in your current theme’s functions.php file:

add_filter( 'pp_subscribe_form_strings_i18n', 'change_pp_subscribe_form_strings' );
	function change_pp_subscribe_form_strings( $strings ) {
		// To change the text "Please wait..."
		$strings['wait_text'] = 'Your custom text...';
		// To change form error "Something went wrong. Please check your entries and try again."
		$strings['form_error'] = 'Your custom text for form error.';
		// To change unknown error "There was an error subscribing. Please try again."
		$strings['unknown_error'] = 'Your custom text for unknown error.';
		// To change validation message for checkbox "Please check the required field.".
		$strings['not_checked'] = 'Your custom text for not checked.';
		// To change empty or invalid email text "Please enter a valid email address."
		$strings['empty_invalid_email'] = 'Your custom text for empty or invalid email.';
		// To change empty name text "Please enter your name."
		$strings['empty_name'] = 'Your custom text.';

		return $strings;
	}