No, it is not a sin to leave and inshallah it is even a rewarded that you even intended to it, and that is by the Qudssi Hadith from Allah [SWT]
Allah has written down the good deeds and the bad ones. Then He explained it [by saying that] he who has intended a good deed and has not done it, Allah writes it down with Himself as a full good deed, but if he has intended it and has done it, Allah writes it down with Himself as from ten good deeds to seven hundred times, or many times over. But if he has intended a bad deed and has not done it, Allah writes it down with Himself as a full good deed, but if he has intended it and has done it, Allah writes it down as one bad deed. It was related by al-Bukhari and Muslim.Source
So inshallah it is rewarded that you even intended and tried, and then it could be that you got the whole reward which is from 10 to 700 Hasnat and that is that if you intended but then you forgot or something actually hindered you from fasting then it as you did it and that is supported by the Hadith from the prophet [pbuh]
Actions are according to intentions, and everyone will get what was intended. Whoever migrates with an intention for Allah and His messenger, the migration will be for the sake of Allah and his Messenger. And whoever migrates for worldly gain or to marry a woman, then his migration will be for the sake of whatever he migrated for. Source
So inshallah your fast is either counted as a whole fast or just the intent but inshallah it is not a sin for leaving it.
Hope that answers your question.
Allah knows best.