Hi Ioana,
You can create year end period as explained by Sachin and Sugandi in previous comments.
However, the period 13 or ‘Year Closing’ periods cannot be used for manual vouchers/entries.
Only transactions posted when executing ‘Clear revenue/Cost Balances’ and and final year end will be posted to these year closing and opening periods.
Therefore, if you want to post adjustment entries for year end, You have to use Ordinary type periods only . (Period 12 most likely)
You can use new voucher type and a user group for this adjustment entries, which will give you some what control.
Hope this helps.
Shehan Almeida.