The method getHttpRequest() does not exist on Omnipay\Common\Message\RequestInterface. It seems like you code against a sub-type of Omnipay\Common\Message\RequestInterface such as Omnipay\IcepayPayments\M...horiseAndCaptureRequest.
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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