This error happens when a collection uses templates and there is no template key found in the document. It is common for this to happen when migrating from fields to templates or during forestry migration.

How to fix

This error can be fixed by adding a root level field of _template to every document with this error. There is not currently any way to automatically migrate the content and it must be done manually or with custom scripts.

Ex: If you had a markdown file you would add:

---
_template: templateName
---

Where templateName is the name of the template in your collection

If it was a JSON file it would look like this.

{
  "_template": "templateName",
}

etc.

Check out this video for a deeper explanation.