NAV 2016 - Creation documents in XML format

Hi all, our customer is interested in creation of customer invoices in XML format. We have seen the possibility to do this through the function "send electronic document -> through document exchange service -> PEPPOL 2.1. When we try to use this function on Cronus database (CH version but also IT version, hf 46045), we have an error message:
The Doc. Exch. Service Setup is missing the secret keys or tokens. Do you want to open the Doc. Exch. Service Setup window?
We have filled in the information in the setup with the function "Set URLs to default", but confirming the setup we have another error message:
The tokens and secret keys must be filled in the Doc. Exch. Service Setup window.
How can we solve the problem?
Thank you,
Elena