To be able to work in a new financial year, it must first be visible to Zenvoices. To achieve this, create the financial year in your accounting package. Then, in your Zenvoices environment, choose Master data full synchronization under the blue button 'Actions' (top left next to the logo). Finally, do not forget to refresh the window (F5).
In short:
Create the financial year in the accounting package.
Fully synchronize the master data using the 'Actions' button.
Refresh the window in Zenvoices (F5).
Please note: perform the steps above before reading in bookings. This prevents you from having to manually select the correct financial year for each suggested booking.