It seems like new Pronamic\WordPress\P...ateways\Ingenico\Data() of type Pronamic\WordPress\Pay\Gateways\Ingenico\Data is incompatible with the declared type string of property $data.
Our type inference engine has found an assignment to a property that is incompatible
with the declared type of that property.
Either this assignment is in error or the assigned type should be added
to the documentation/type hint for that property..
The expression return $this->data returns the type string which is incompatible with the documented return type Pronamic\WordPress\Pay\Gateways\Ingenico\Data.
Our type inference engine has found an assignment to a property that is incompatible with the declared type of that property.
Either this assignment is in error or the assigned type should be added to the documentation/type hint for that property..