Cron job fails with Error 500 Internal Server Error
 I receive the following error 

```
Status: 500 Internal Server Error
X-Powered-By: PHP/5.6.31
Set-Cookie: PHPSESSID=xxxxxxxx(I replaced the value); path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-
check=0
Pragma: no-cache
Content-type: text/html; charset=UTF-8

```

I have tried changing the cron.php file permissions to 744, 755, and 644 respectively but still does not work. I set up the cron with the command

```
/usr/bin/php /home/my_username/public_html/cron.php

```

* Does anyone know how I can fix this?