"this is all I know about Kyiv."
Translation:Це все, що я знаю про Київ.
I entered "це все я знаю про київ" and did not have "що". Translated literally the answer is " This is everything what I know about Kyiv." Why is the "що" needed?
Yes, it is. With «що», this is a complex sentence with a subordinate clause ('this is all' is the main clause, 'I know' is the subordinate). Without «що», this will be understood as a single simple sentence: «це все я знаю про Київ» will be understood to mean 'I know all this about Kiev' (i.e. «це все» will be an understood as an object, because it can be either nominative or accusative).