Found my error.
Need to give an alias after my close bracket of UNPIVOT
UNPIVOT
(
[Postings] FOR [Periods] IN ([Balance Forward],[Net Posting 01], [Net Posting 02], [Net Posting 03], [Net Posting 04], [Net Posting 05], [Net Posting 06], [Net Posting 07], [Net Posting 08], [Net Posting 09], [Net Posting 10], [Net Posting 11], [Net Posting 12] )
) as unpvt