Fiscal period is not open

I found the issue 'Fical period 5/5/2016 is not open' when I tried to invoice a purchase order yesterday. I solved it by this post Error - Fiscal period for xx/xx/xxxx is not open.

Above picture is captured from the Internet because I forgot to capture mine ;(

Solution
General ledger -> Setup -> Ledger  

Click  'Ledger calendar'..  make sure that the period you're going to process, the period status is set to 'Open'.