Make sure you have one of the following Dynamics 365 Business Central versions installed:

NAV 2016 as of CU55 (05/20)
NAV 2017 as of CU41 (04/20)
NAV 2018 as of CU28 (04/20)
Business Central 13 as of CU18 (04/20)
Business Central 14 as of CU11 (04/20)

The XRechnung module generates invoices and credit notes in XRechnung format and checks whether the generated XRechnung files are compliant with the formal specifications. This check is done by default using a validation tool which is located on an external server. This means that requests must be sent over the Internet to this corresponding server.
The server “listens” to requests via port 5090. If you want to use our validator, you have to make sure that outgoing requests are allowed to be sent via port 5090. You can make a corresponding setting in the “Firewall settings.
Alternatively, it is possible to perform the validation through your own set up validation server.