Some people like to put apostrophes into words where they aren’t necessary, but others don’t want to be bothered with the extra keystroke. If you leave the apostrophe out of ill, it will look sickly. 🤪
I’ll: (subject pronoun + verb) contraction of I will
ill: (adjective) unwell, sick;
(adverb) badly, hardly;
(noun) harm, misfortune