Russian keyboard has no Ukrainian "i" - why don't you accect a "и" ?- or accept an English "i" instead of marking the sentence wrong.
i dont know in detail why, but the coding for an english i is different than the coding for a ukrainian і. I think и isnt accepted because it is technically a spelling error and it migth work some of the time but not all the time. I would suggest installing a ukrainian keyboard seeing as you are level 14, you've probably put a lot of time into this. on phones you can add one the same way as getting an emoji keyboard but choosing ukrainian instead, and switch between them like you switch to emojis. on most PC's you can get one in settings by searching keyboard in the search bar and finding the one you want. hope this helps a little!
Because the Ukrainian alphabet is different than the Russian one. Because the words with "і" or "ї" aren't spelled with "и". Would you spell "cat" like "cæt" simply because the Danish alphabet has the letter "æ"? It doesn't make sense. If you need a Ukrainian keyboard, there is one online here.
There's a few more letters in Ukrainian other than just "і" that don't exist in Russian. Accepting "и" is not a good solution, as it's an entirely different letter in Ukrainian, which sounds closer to the Russian "ы". The idea of trying to accept letters that just look similar is troublesome, too, as they aren't the same letters. Furthermore, it would still leave the question of the letters like ґ, є, ї and the apostrophe (') as well as the Hryvnia sign (₴), since those don't exist on the English layout, either. Add the Ukrainian keyboard layout on your PC to solve this. The keys on the Russian keyboard coorrespond to Ukrainian ones as follows:
ы = i
ъ = ї
AltGr+г = ґ
э = є
ё = '
Additionally, I have developed a small project that allows you to retain two keyboard layouts on a Russian system rather than having to deal with three of them, which gets annoying as all hell, admittedly. See https://github.com/tlm-2501/ruukrkbd. This is for Windows only, but from what I know, creating a keyboard layout in Linux-based systems is pretty much a breeze so you could replicate it.