If you want to be able to accrue your journals or supplier/customer invoices, you first need to activate it under Settings - Accounting - Accruals.
Here you can set up a lowest amount that can be accrued in the field Lowest Accrual amount (EUR). Use the check boxes to select if you want to be able to accrue invoices inclusive of VAT. This is available for supplier invoices as well as for customer invoices. Do not forget to Save when you are done.