If you are using 12 period accounting and you wish to use the nominal adjustment period (13), the best way to approach this is as follows:
Year end the sales ledger at period 12
Year end the purchase ledger at period 12
Period end the nominal ledger at period 12
This will place you into the following scenario:
New sales and purchase transactions will go into nominal N1. Nominal period 13 will be open so that adjustments can be made.
Note that adjustments are exactly that - you can create nominal journals that move money between nominal account A and nominal account B, but you cannot create sales or purchase ledger transactions, nor can you post anything that affects customer or supplier balances.