Hi Watsojanh,
We apologize for the delay in response.
As shared, the error message appears as "These documents have security settings which prevent comparison. Please modify security settings to allow editing and try again." which indicates that the "Editing" for these documents has been restricted.
What you are looking at under the document properties is "Security Method". So you need to check the "Document Restrictions Summary" and see whether it is allowed or not?
Sample Screenshot-

For more clarity, you may attach the screenshot of the "Security" tab for both the documents.
https://forums.adobe.com/docs/DOC-7043#jive_content_id_How_do_I_attach_a_screenshot
Hope this helps.
Regards,
Akanchha