In Billic admin area at Settings » BitPay you need to set your "BitPay Encrypted Data".
You can find this in your BitPay account by going to Payment Tools » Payment Button.
Fill in the following Info;
Default Price: 100 EUR (or your Billic currency)
Server IPN: https://www.YOUR-WEBSITE-URL.com/Gateway/BitPay/
Browser Redirect: https://www.YOUR-WEBSITE-URL.com/User/Invoices/
...then click generate.
You will be given a HTML form where you need to extract the encrypted data string from and put this into the Billic settings.