Hi there,
Welcome to the Community. I'll be happy to share info about making a supplier prepayment.
Based on what you've described, you'll need to make a payment to the supplier. The reason I suggest this is that you don't need to have a bill to make a payment to the supplier. Once you've entered the payments, you can always attach a bill to it once you've created it.
Let me know if this info helps. If you need further assistance, don't hesitate to contact our support team using this link here.