systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units
Normally when I've seen this it's when you are using a variable like a normal string when in fact it's actually an array such as this example:
[Tue Mar 13 04:22:35 2018] [error] PHP Catchable fatal error: Object of class WP_Term could not be converted to string in /vhost/httpdocs/wp-content/plugins/wp-instagram-post/classes/class-woo-igp.php on line 578
I modified this code after quickly learning how Wordpress plugins actually work, how they're called etc.. was the first trick to modifying the code to add tags.
Add this code after line 570 in wp-content/plugins/wp-instagram-post/classes/class-woo-igp.php