Mistake after Update

Hi,

I just executed the new update. After finishing installing, I see an error message at the top of our website that says

“Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘wpseo_frontend_head_init’ not found or invalid function name in /www/htdocs/w01080e8/treditionwp/wp-includes/plugin.php on line 429”

What do I have to do to solve the problem?

Regards,

Sandra