Could not recognize the tag for the optimization of the audience

Question

I've pubblished my rss feed for facebbok instant articles. When I test my url in Instant-Article --> Configuration --> debug feed rss I see for any article the following warning message "Could not recognize the tag for the optimization of the audience. Update the item with new tags and try again". I'm using Facebook audience network, with one active position.

I've set
<meta property="fb:use_automatic_ad_placement" content="true">

and

<figure class="op-ad"><iframe width="320" height="50" style="border:0; margin:0;" src="https://www.facebook.com/adnw_request?placement={my_id}&amp;adtype={type}"></iframe>
</figure>

for every article. In the official documentation I didn't find nothing of relevant about this message. Can someone say me where I'm wrong?

Answer 1

I found the solution in Facebook Instant Article documentation: I was passing the op:tags meta empty in this way:

<meta property="op:tags" content='@Model.GetTags()'>

but Model.GetTags() was giving me empty string, after the change in this method the error disappeared immediately.