It's fine. Did you report that your answer should have been accepted?

It is accepted now.

去年 is very common. Though 上年 is a word, I don't recall the last time I actually heard someone use it in everyday speech. 上 as a relative time word much more commonly applies to months (上个月) and weeks (上个星期) and, of course, occurs in the word 上午 'A.M.'.