Feedme Skip Entry if No Matching Elements on Field
isValid should be set on the $event variable, not on $event-&gt;element:
Event::on(Process::class, Process::EVENT_STEP_BEFORE_ELEMENT_SAVE, function (FeedProcessEvent $event) {
    $myField=$event-&gt;feedData['myfieldname'];
    if ($myField == null){
        $event-&gt;isValid=false;
    }
});

I haven't tested but looking at the feed-me source code, this should work.