Unfortunately, once you run the year end process and choose to move to the next financial year, leaving the previous year open for adjustment journals only, or just closed altogether, then period 12 of the previous year cannot be re-opened.
If you wish to keep period 12 of the previous financial year open then you MUST choose the third option, move to the next financial year keeping period 12 open for normal postings.
There are several warning messages to stop you closing period 12 by accident.