Hi @shaunf and so sorry for the snag.
This “no metadata” error message is actually caused by a bug that affects multiple Atlassian add-ons. Atlassian support is investigating it and you can follow along (and vote for it) on this page.
In the meantime, would you like to work on Mockups 3 for Desktop?
It uses the same editor and file format that Mockups 3 for Confluence Cloud, and comes with a 30-day free trial. You can download it on this page.
I can also give you a Mockups 3 for Desktop trial extension in case you’ve already used a trial, just let me know via firstname.lastname@example.org.
Sorry again for the trouble. Please let us know if you need anything else, we’re here to help!