The method initialize() does not exist on Behat\Behat\Context\Context. It seems like you code against a sub-type of Behat\Behat\Context\Context such as Knp\FriendlyContexts\Context\Context or Knp\FriendlyContexts\Context\RawMinkContext.
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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