Account period closed

Seeing this error means only one thing: you have turned on the ‘Closing books’ option in your QuickBooks Online account.

If ‘Closing the books’ is turned on, you can set a closing date to limit changes to company data. Then, when anyone (for example Dext) tries to save a change to data that is dated prior to the closing date, a warning message appears.

Perhaps here you might find more useful information.