There are three fields of primary interest here:

1.  Current period: This controls which period will show by default on your reports and is used as a starting point for the Future open periods. If you cannot make entries in the calendar month you are in you most likely need to change the Current period to make it more current.

2. Future open periods:  The number of periods that are added to the Current period.

3. Last period closed:  This controls how far back in time you can make entries.  If you need to make an entry in the past and the period you need to make an entry in is not available this period needs to be set back to the period before the period you need to make an entry in.