There are karma thresholds that unlock certain features, an example being able to downvote a comment. There might be some mechanism in place that prevents new (or not active yet) users from submitting URLs. You had no prior comments or submissions here and also had not gained any karma yet. Maybe try again after participating here for a while.

You can find undocumented features of HN here: https://github.com/minimaxir/hacker-news-undocumented