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This error means that the computer is no longer able to resolve your local hostname. 
This is due to the fact that the 
/etc/hostname
 file is not the only file where 
your hostname is located; it's also referenced in 
/etc/hosts
. Unfortunately, the 
hostnamectl
 command doesn't update 
/etc/hosts
 for you, so you'll need to edit that 
file yourself to make the error go away. Here's what a 
/etc/hosts
 file looks like on 
an example server:
Figure 10.2: Sample contents from a /etc/hosts file
The first two entries, in this example, refer to the local machine itself. Localhost 
addresses, also known as 
loopback addresses
, allow the machine to essentially reach 
itself. If you were to use the 
ping
 command against the 
127.0.0.1
 address, the reply 
would come from the machine that executed the command, not from another host on 
the network. On the first line, we have the following:
127.0.0.1 localhost
If you were to use any networking command to attempt to communicate with the 
local server, such as pinging 
127.0.0.1
 or 
localhost
, the 
/etc/hosts
 file on this line 
declares that this communication is directed toward the underlying server itself.
With the second line in the example screenshot, we have the following:
127.0.1.1 dev.mynetwork.org dev
Depending on your configuration, such as whether you are using a physical server, 
a virtualization platform, or a cloud server provider, that line may or may not be 
present. You can add that line if it's missing, but we'll talk about that more in a 
moment. 

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Chapter 10