When I tried answering Yes that that prompt all it did was take me back to the Change Accounting Period window, presumably so I could cancel it and go print checks or invoice. Just answer no to this prompt.

By the way, you don't have to change periods. All transactions will post based on their date (invoice date, check date, etc.) regardless of what the current period is set to when you enter them.

You can enter any transactions into Sage 50, so long as those transactions are within the 2-year open period. If you are trying to enter transactions for a closed period, you cannot enter the transactions. I do not know of a way to be able to enter transactions into a closed period (a year that has been closed out already). I suppose you could restore that particular year to enter the transactions, however, I do not know whether or not the entered transactions (in the closed period) will transfer into the actual "open" years where most of your transactions are being entered.

I had a client that had two old transactions (vendor bills from 2017) that had not been entered. I started using Sage 50 earlier this year, and simply brought forward opening balances from 2018 (for this client). I did not realize that there were a couple transactions from 2017 when I brought the opening balances into Sage 50. I had already closed out 2018. (2017 was never an "open year.) I ended up having to enter the 2017 transactions as of 01/01/2019, making a note that the transactions were originally from 2017. The client paid those two transactions in September, 2019.