Colin might be right, but there could be other reasons for backups throwing that error ... which isn't a Moodle error but an Apache server/setup error.
In moodle, go to Site Admin -> Server -> PHP info
Two lines:
System and Server API
What do they say?
Your server appears to be hosted with Linode. They offer many different flavors/setups ... example: operating systems: CentOS 6 and 7, Debian, Ubuntu ... as well as web services ... nginx, apache2, and lightspeed. etc.
While they are all Linux, there is enough of a difference such that one needs to find Linode's docs that are specific to your setup.
Perhaps your best resource for setup of servers on Linode is this:
https://linode.com/docs/web-servers/
I don't host with Linode ... so if we start asking specific questions as to how Linode does things you are much better off contacting Linode tech support or browsing Linode forums.
'spirit of sharing', Ken