What does the VAST error " must contain at least one " mean?

Wrapper-level impression beacons are fired in addition to those in the eventual ` `. At least one is required so the intermediary ad server can record its impression.

VAST-2.0-wrapper-impression

<Wrapper> must contain at least one <Impression>

ErrorRequired fieldsIAB VAST 2.0 §2.5VAST 2.0 and later (all versions)

Wrapper-level impression beacons are fired in addition to those in the eventual `<InLine>`. At least one is required so the intermediary ad server can record its impression.

How to fix

Add <Impression><![CDATA[https://track.example.com/impression]]></Impression> inside <Wrapper>.

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Other Required fields rules

VAST-2.0-root-elementRoot element must be <VAST>VAST-2.0-root-version<VAST> must have a version attribute VAST-2.0-root-version-valueVAST version attribute must be a recognised version string VAST-2.0-root-has-ad-or-error<VAST> must contain at least one <Ad> or <Error>VAST-4.0-wrapper-root-error<VAST> root contains both <Ad> and <Error> elements (invalid per VAST 4.0)VAST-2.0-ad-has-inline-or-wrapperEach <Ad> must contain exactly one <InLine> or <Wrapper>VAST-2.0-inline-adsystem<InLine> must contain <AdSystem>VAST-2.0-inline-adtitle<InLine> must contain <AdTitle>