A conversion that failed
Send the video URL and the exact message the page showed. Include whether the video plays in your browser and whether you were signed in, because those two facts separate a region lock from a permission problem and they are what the troubleshooting page is built from.
A figure that is wrong
Send the page and the sentence. Size and bitrate figures are computed rather than typed, so a wrong number usually means a wrong bitrate setting rather than a typo — which is worth knowing even when the correction is small.
A copyright concern
This site publishes no audio and hosts no catalogue: files exist only for the length of a retention window and are deleted on download. A concern about audio converted through this service is best raised with the platform hosting the source video, where the upload and its rights are — the takedown page sets out what a notice needs and where each kind of request belongs.