Ahh, I see. Once you have opened your next year, you can no longer post to prior year. This is not a security setting but the way it works. You cannot post to prior, current and next year all at the same time.

If you have recently opened the next year, then perhaps you can restore your data from a older backup before creating the Next year, then re-do the work for posting to Prior Year, then open your Next year.

If it has been some time since opening the Next Year, and re-doing work is not palatable, your only other choice is to use the Import Transactions function to get the entries into the prior year. Note that you also have to account for Retained earnings manually if you choose this option.

It seems pretty late to have adjusting entries for Prior Year, over a year later so that the Next Year needs to be opened. Is there a reason why these entries are so late, and what did you do before Ledger 90A allowed opening a Next Year before closing the Current year?