The second one –
chan eil e bliadhna fhada – is definitely not correct, as you cannot use chan eil with a noun phrase predicate.
But chan e bliadhna fhada a th’ ann is fine.
As for why the second is incorrect – it’s the same difference between a substantive verb bi (tha, chan eil) and copula (is, cha(n)) as explained in the "It is another sandwich" sentence.
EDIT: and you can now read much more about it in my Guide to Scottish Gaelic ‘to be’, the linking verbs: substantive ‘bi, tha’ & the copula ‘is’: https://forum.duolingo.com/comment/45459577