Set JetFormBuilder form statuses. All credits to @girafffee

jet-fb-form-statuses-hook.php

<?php

use Jet_Form_Builder\Exceptions\Action_Exception;

add_action( 'jet-form-builder/custom-action/test_action', function ( $request, $handler ) {
	if ( empty( $request['age'] ) ) {
		/**
		 * You can use one of the default statuses
		 * 'success' => 'Form successfully submitted.',
		 * 'failed' => 'There was an error trying to submit form. Please try again later.',
		 * 'validation_failed' => 'One or more fields have an error. Please check and try again.',
		 * 'captcha_failed' => 'Captcha validation failed',
		 * 'invalid_email' => 'The e-mail address entered is invalid.',
		 * 'empty_field' => 'The field is required.',
		 * 'internal_error' => 'Internal server error. Please try again later.',
		 * 'upload_max_files' => 'Maximum upload files limit is reached.',
		 * 'upload_max_size' => 'Upload max size exceeded.',
		 * 'upload_mime_types' => 'File type is not allowed.',
		 */
		throw new Action_Exception( 'empty_field' );
	}

	if ( absint( $request['age'] ) < 18 ) {
		throw ( new Action_Exception( 'Your age is less than necessary' ) )->dynamic_error();
	}

	/**
	 * If all checks are passed, you just need to do Nothing,
	 * so that the form would continue its work or successfully complete it.
	 *
	 * In rare cases, you can interrupt the execution of the form with a successful status.
	 */
	if ( 199 === absint( $request['age'] ) ) {
		// or throw new Action_Exception( 'success' );
		throw ( new Action_Exception( 'Lucky!' ) )->dynamic_success();
	}

}, 10, 2 );