Hi David,
The issue seems to be communication with gateway service on ERP end.
Please follow the guide:
to send IDOCs to MII. Though this was written for MII 12.1, it is valid for 15.0 as well.
You will see how to find parameter values within the guide. Also you need to configure MII listener at MII end before you do a connection test.
Regards,
Anushree