Thank you for following up with this information @nathanu.
It looks like your organisation has multiple Confluence Cloud instances, and it seems like this document is located in a Team folder which parent folder is owned by an Enterprise account that you are not part of (although it is still under your organisation’s domain).
Unfortunately, a Confluence instance must be linked to a single Lucid account and only users on that Lucid account can embed Lucidchart documents inside Confluence. In this case, this document can be embedded by a user who is part of a Lucid Enterprise account linked to their Confluence Cloud instance.
I am sorry for the inconvenience, please let me know if you have any additional questions or concerns!
Cheers