Once you have Started a New Fiscal Year, you cannot make adjusting entries into the previous fiscal year.

You can either restore the pre-close backup and start over from that point, or you can make an adjustment as of the first day of your new fiscal year.

NOTE: Speak to your accountant if this situation arises, they may be able to suggest alternative solutions or provide general journal entries you can post to make prior-year adjustments to your general ledger.