The method getContentString() does not exist on League\CommonMark\Parser...ContinueParserInterface. It seems like you code against a sub-type of League\CommonMark\Parser...ContinueParserInterface such as League\CommonMark\Parser\Block\ParagraphParser or League\CommonMark\Parser\Block\ParagraphParser.
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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