If the only Fiscal Year option you have in Voucher Entry/Check Processing is the current fiscal year, you have set the system to disallow creation of GL transactions to the previous year.  To remedy this, close AP Manager.  Open AR Manager and choose TOOLS, System Settings.  When the System Settings window appears, place a check mark next to the “Allow GL Transactions to be created for the Previous Year?” option. 

Click OK to save your changes, go back into AP Manager and enter your voucher/manual check for the previous year.

To prevent mistakes from happening further down the line, you may want to go back into System Settings and remove the check mark from the option once you are done.