Fatal error after installing AutoFocus

After installing AutoFocus+ to blog.karenjohanson.co exactly as described in the documentation at https://fthrwght.com/autofocus, I got the “Fatal error: Call to undefined function wp_enqueue_style() in /home/bikegrrl/blog.karenjohanson.com/wp-content/themes/autofocusplus/functions.php on line 625.” Any idea why this is happening? I switched back to the old AutoFocus theme until this is resolved.