add bellow code in function.php file

function getPostViews($postID){
    $count_key = 'post_views_count';
    $count = get_post_meta($postID, $count_key, true);
    if($count==''){
        delete_post_meta($postID, $count_key);
        add_post_meta($postID, $count_key, '1');
        return "1";
    }
    return $count.'';
}
function setPostViews($postID) {
    $count_key = 'post_views_count';
    $count = get_post_meta($postID, $count_key, true);
    if($count==''){
        $count = 0;
        delete_post_meta($postID, $count_key);
        add_post_meta($postID, $count_key, '1');
    }else{
        $count++;
        update_post_meta($postID, $count_key, $count);
    }
}

add in theme where you want to disply

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