openrtb.bid.mtype_not_offered
What it means
The bid's mtype declares a media type the referenced Imp never offered: a banner bid (mtype 1) against an imp that only carries a video object, and so on. The offer defines what may be sold; the bid stepped outside it.
Why it matters
The exchange discards the bid, because delivering a banner into a video slot is not a rendering option. Systematic mismatches point at campaign targeting keyed to the wrong media type or an mtype constant baked into one code path.
How to fix it
Bid only the media types present on the referenced Imp (banner, video, audio, native). If a campaign can serve several formats, choose the mtype after selecting which of the imp's offered types you are answering.
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.