Is bonfartis a viable word?
can "cxu sxi sentis bone hieraux" work?
and if it cant why not...sento is a noun based word?
No, that won't work, because ‘senti’ is reflexive. What would work is:
Ĉu ŝi sentis sin bone hieraŭ?
I put ĉu hieraŭ ŝi fartis bone.
I don't really understand why that is incorrect. Can someone with more Esperanto experience explain it to me?
It isn't; your sentence is perfectly fine! You just chose a word order they hadn't thought of, wherefore it is not yet amongst the correct answers. Next time you come across this question you can report it and it will (eventually) be accepted ;).
Where is a typo in my "cxu sxi fartis bone hieraux"?
There is none; that sentence is correct.
Why is the word fartis considered correct and sentis is not.
Literal translation to Croatian would be "Ĉu ŝi sentis sin bone hieraŭ" and it's accepted!