"The boy cannot eat fish."
Translation:Gutten kan ikke spise fisk.
Yes, the adverb should come after the first verb, 'kan', so yours would be grammatically incorrect.
how would one say "the boy cannot be eating fish!", as in an expression of disbelief? I think I find it difficult to understand that there is no distinction between "the boy eats" and "the boy is eating" in Norwegian, and wonder if this extends to "can eat" and "can be eating". Thank you.
I am very confused by "Gutten kan ei ete fisk." I don't get it. Why not spise?
That's Duo failing to distinguish between possible translations and ones it should suggest.
Ei and ete are dialectal variations on ikke and spise respectively. They oughtn't to be the first choice of word to go for in Bokmål.