Sunnah is not obligatory so one is not sinful for not praying them.
However, you will lose a great deal of reward for not praying it and also your Fard prayers have a lot of errors in them that are only compensated by sunnah prayers!

We know that there are 17 units of prayer that are obligatory (فرض) in one day. 2 in fajr 4 in zuhr 4 in asr 3 in maghrib 4 in Isha . What if a person prays only these 17 units?? Will he be asked about the remaining (Sunnah) units which he didn’t prayed??