Heroin and heroine are homophones and easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Heroin is a noun. It means a powdery drug made from opium plants that is highly addictive.
Heroine is a noun. It means a female central character in a story, or a female rescuer in a story.
The following story uses both words correctly:
Hereswitha was a heroine of her local town. It had been plagued by economic troubles and growing heroin addiction by local townfolk, but her leadership turned things around.