To configure the accounting export, go to "Settings > Accounting > Accounts" and enter all accounts used. You also have the option of whether you want to use debitor accounts, whether you want to use deferrals automatically on a monthly basis, and whether you want to output for each invoice item. You can include payments with the respective posting accounts and decide for each country whether you want to show your revenues as gross or net with the respective tax accounts.
You have to define all used accounts, otherwise accounting export cannot be carried out and you will get an error message due to unconfigured accounts. All fields must be filled before the next export.