If the Lateral Similar Pages add-on says it does not have access to a page it may be due to a couple reasons.
One is that Lateral doesn't have access to the Space that the content is within. To fix this, follow the instructions here.
The other may be that Lateral does not have permissions to view the page. In this case thepage/blogpost will have a red lock. To give Lateral permissions for this page/blogpost please follow these instructions.
Lateral can also be given permissions by being made part of Confluence groups, more information on groups and how to manage them is provided by Atlassian here.
On the page/blogpost click the red padlock icon near the title:
In this window, where it says 'Type a user name' enter 'Lateral':
Click 'Lateral' and the click 'Apply':
Now wait a few seconds and the Lateral Similar pages add-on should have access to the content.