This error is most probably due to:

1.  Running an old PHP version on your hosting / server.  In this case you have to upgrade or ask the hosting company to do that.

2. Theme files have been changed. In this case you have to remove your changes or debug them.

The theme and plugins require PHP version 7.0 or higher. Now is the time to think about upgrading to PHP 7.0 if you haven’t already. Not only because you want to be running on supported software, but you are guaranteed performance benefits!  If your current WordPress host doesn’t support PHP 7.0 yet, we strongly encourage you to seek out new hosting.