The method addField() does not exist on Solarium\Core\Query\DocumentInterface. It seems like you code against a sub-type of Solarium\Core\Query\DocumentInterface such as Solarium\Plugin\MinimumScoreFilter\Document or Solarium\QueryType\Update\Query\Document.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation