Hi,
Sorry for the issues you are having. I can help troubleshoot this with you.
It looks like that warning is because our plugin does not generate a offer without tickets.
Although it is generating a warning and is recommend by Google it is not required:
offers (optional) – An offer to buy tickets for this event. If present, some subfields are required.
https://developers.google.com/structured-data/rich-snippets/events
I looked into how the plugin works and we do add offers if using Event Tickets Plus, I checked against the structured data tool and it worked correctly with that plugin and an event with tickets.
We have a filter and I created a quick snippet to fill in the offer field:
https://gist.github.com/jesseeproductions/f022b8d8b4a9fa03de21
It could be expanded on to do more, but this will return a valid offer if you fill in the price.
You could add more custom fields using a plugin such as ACF to add in specific information or each events.
However, that is beyond the support we can provide on this, beyond answering a couple questions.
Thanks