Ask a supplier for a missing invoice

Open Finances and then the Accounts tab. Scroll to the Month-end check, which lists everything still open for one month. Choose the month in Month.
The list Category bookings without receipt shows bank payments you made that were booked to a category but have no receipt behind them.
Click Draft request on the payment whose invoice is missing. The agent writes a short, polite letter to the payee asking for the invoice.
The draft shows the recipient if one is known, a subject and the text. It says Only a draft. Nothing was sent.: the app never sends it for you. The letter is written in German, whatever language the app is set to.
Click Copy, paste the text into your own mail program and send it from there. Click Close to hide the draft.
Upload the invoice when the supplier replies, see Upload a receipt or invoice.