The expression return $this->setData(Ge...RD_NUMBER, $cardNumber) returns the type Getnet\PaymentMagento\Model\Data\NumberToken which is incompatible with the return type mandated by Getnet\PaymentMagento\Ap...erface::setCardNumber() of void.
In the issue above, the returned value is violating the contract defined by the
mentioned interface.
Let's take a look at an example:
interfaceHasName{/** @return string */publicfunctiongetName();}className{public$name;}classUserimplementsHasName{/** @return string|Name */publicfunctiongetName(){returnnewName('foo');// This is a violation of the ``HasName`` interface// which only allows a string value to be returned.}}
In the issue above, the returned value is violating the contract defined by the mentioned interface.
Let's take a look at an example: