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 the page has restrictions on it, if this is the case there will be a red lock on the page, you can give Lateral access to the page by clicking this lock and giving it permissions. To fix this, follow the instructions here.
The other may be that Lateral does not have permissions for the space the page is in. In this case the page will have an unrestricted open lock. To give Lateral permissions for this space 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.
Click 'Space Tools' in the bottom left of the page or blogpost that is within the space:
Now choose permissions:
On this page, under the 'Individual Users' heading, click 'Edit Permissions':
On this page, under the 'Individual Users' heading again, there is an input box, enter in to this 'Lateral':
Click the heading that says Lateral (addon_....). Please note the content of the brackets will likely be different. Now click the 'Add' button. The page will reload and you will see that the Lateral user now has 'View' permissions for the space:
Important: Click the 'Save all' button at the bottom of the page otherwise the update will not be triggered: