I'm attempting to create a subpage but I'm running into the error mentioned in the title. I used fiddler to get the request being sent and it looks like the following.
POST https://sites.google.com/feeds/content/domain/site/ HTTP/1.1
Content-Type: application/atom+xml; charset=UTF-8
User-Agent: G-google-SitesAPIDemo-v1.2/GDataGAuthRequestFactory-CS-Version=2.1.0.0
Authorization: xxx
GData-Version: 1.0
Host: sites.google.com
Content-Length: 17272
Expect: 100-continue
<?xml version="1.0" encoding="utf-8"?>
<entry xmlns="http://www.w3.org/2005/Atom" xmlns:gd="http://schemas.google.com/g/2005">
<sites:pageName xmlns:sites="http://schemas.google.com/sites/2008">Title</sites:pageName>
<title type="text">title</title>
<link href="https://sites.google.com/feeds/content/site/domain/Parent_Id" rel="http://schemas.google.com/sites/2008#parent" type="application/atom+xml" />
<category term="http://schemas.google.com/sites/2008#announcement" scheme="http://schemas.google.com/g/2005#kind" label="announcement" />
It matches up to the api for doing so but I'm not sure what seems to be the issue.
I had mistakenly switched my domain and site in the parent link. Fixing this fixed the issue.