openrtb.bid.adm.native_not_json
What it means
The bid declares mtype 4 (native), but adm does not parse as a JSON Native Markup Response. Native creatives travel as structured JSON assets, not as HTML or XML.
Why it matters
The publisher's native renderer receives assets it cannot parse, so nothing renders. The declared type and the payload disagree, and one of them is lying; either way the impression is lost after the win.
How to fix it
Return a valid Native Markup Response (JSON, matching the ver requested) in adm when bidding mtype 4. If you meant to send a banner or video creative, declare the mtype that matches the markup instead.
Check your payload
Paste a bid request or response into the tester to see whether this code fires against it, or gate on the id in CI with the CLI: the id is stable, wording is not.
Back to the full diagnostic code reference.