API failure: This server is already a child node

Symptoms

When using the Link Server Nodes functionality in WHM, the following error was provided:

The "PRIVATE_set_as_child_node" request to "child.hostname.tld" as "root" failed because of an error: API failure: This server is already a child node.

Where child.hostname.tld is the hostname of the child node you are attempting to link.

Description 

When a child node is successfully linked to a parent node, a touch file is created at:

/var/cpanel/is_child_node

This file defines the server as a child node.  If the server node link is broken between the parent and child node, this file may remain on the child node causing issues when attempting to re-establish the link between the servers. 

Workaround

Note: A child node can only have a single parent node; if the child node is already successfully linked to a parent node then do not remove this file.

Remove this touch file from the child node and attempt to link the servers again:

rm /var/cpanel/is_child_node