Sage only allows entries in 2 fiscal years - the current fiscal year and the immediately prior one.  What matters here is the current and prior fiscal year according to Sage.  If you look at Setup- Settings - Information, there will be a box at the top right labelled 'current year dates' and one right under it labelled 'last year dates'.  The Fiscal Start date in the Last Year Dates box will be the earliest date you are able to make entries for.

This is a program limitation - the 'do not allow entries before x date' tick box can make the earliest entry date LATER than last year's fiscal start date, but nothing can make it EARLIER.

If your data set as been rolled forward into a new fiscal year, there are only two ways I know of to fix the problem.  Unfortunately, neither way is nice, so you'll need to choose the least painful.  You may want to consult your accountant when making the choice.

Choice 1:  Find the backup that was done when the data set was rolled into the year that's currently labelled 'current year dates'.  Restore the backup, and re-enter everything that was entered from that date forward.  If you choose this option, you'll probably hold off on opening up the next year until you have the 2019 entries from your accountant.

Choice 2:  Keep on with the current data set, but work with your accountant to identify entries that you have made in 2021 that should be reported on the 2020 financial statements / tax return.  Your accountant will likely give you various entries to make that adjust the opening retained earnings account.