上年

上年 is more formal than 去年 (qùnián, 'last year') and is the standard term in financial reports, accounting, and statistics — phrases like 上年同期 ('same period last year') and 上年度 ('previous fiscal year') are everywhere in business writing. In casual speech, just say 去年.

mistakes

Don't confuse 上年 (last year) with 上半年 (shàngbànnián, 'first half of the year') or 早年 (zǎonián, 'in earlier years / long ago'). The 上 here means 'previous', the way 上周 means 'last week' and 上个月 means 'last month'.