"tosi" is both "very" and "really. "tosi" is actually just a shortened version of "todella" and it isn't formal language, but it's used a lot in spoken Finnish compared to "todella" which is more formal
If Duolingo doesn't accept one of the words, but does accept the other one, you should always report it!
From wiktionary: tosi is the nominative, while todella is the adessive case. Tosi has more cases and it's usage is a bit different from other words. Read the page on tosi in wiktionary: https://en.wiktionary.org/wiki/tosi#Finnish. It shows a bit more complexity and nuance than the other answers I have seen here.