"When will she go home?"

Translation: वह घर कब जायेगी?

[NatNomNom]

Why is it not veha kab gher jayegi?

[luna817570]

Why can't I start the sentence with kab?

[itsmeyash31]

In colloquial speech it can be. Note, Hindi is a word order free language (with the default word order being SOV).

But, since Duolingo follows the default word order. कब must be before the verb of the sentence.

In colloquial speech you would hear question words like कब be placed anywhere in the sentence, depending on how the sentence/question is to be emphasised.