If you get this error message you are trying to post a transaction in a period that is either 'inactive', 'blocked' or 'closed'.
Go to General Ledger/Maintenance/Accounting year and select the accounting period that you wish to post in.
Click on the 'Period status per month' button.
The status is set to 'inactive' by default. This means that you will not be able to post in future periods but only up to todays date.
If you want to post in future periods then you will need to change the period status from ' inactive' to 'active'.