Thanks for getting back to us Lisa, this is a common occurrence for customers who have migrated from Desktop, we've got a full guide linked here for reference. Please get back to us below if you have any trouble actioning this :)
Edit- Please also check that you don't have the 'close the books' setting enabled as this will prevent you from being able to make changes to transaction prior to the date set - you can manage this by navigating to the Cog > Account and settings > Advanced > Accounting
