One solution is right there in Kern's post.
But this is a PHP error and nothing to do with aapanel.
I do not know Sendy but take a look at the code:
Call to undefined cal_days_in_month() in /www/wwwroot/sendy/index.php:153
So it is calling a function that is not there = cal_days_in_month.
If Sandy requires the php calendar then follow Kern's instructions.
It is very easy to just google = how to install php ext calendar on linux...