If I had a Neyah to fast one of the 6th days of Shawal, and for no crucial reason i didn't, what do I ahve to do? keeping in mind that i don't have any outstanding days from Ramadan

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Whoever intends to fast one day optionally, whether one of the six days of Shawwaal or else, but he fails to do so, or that he started fasting and then broke the fast, then he is not obliged to make it up and he is not sinful Allaah Willing, but he has missed out on a great reward.

A person who fasts optional fasting may do as he wishes; he may complete the fast or he may break the fast as confirmed from the Prophet .