Problem Statement
When opening UA Configuration tool and choosing "Import Certificate to Trust" on "Manage Security" tab, the following error appears

this means that a Config.file has not been added to the Opc.Ua.Discovery.Server

Solution

1. First make sure the services Aspen CIM-IO Manager and OPC UA Discovery Server are running on the same administrator accounts.
2. On OPC UA Configuration tool | Manage Security, you need to have Opc.Ua.DiscoveryServer on Application To Manage, click Edit

a window will pop out, click Browse for Configuration File 
3. Go to ProgramData | OPCFoundation | Config and open the Opc.Ua.DiscoveryServer.Config.xml


and select the .xml file 

4. Click OK


Now you will be able to import certificates to trust.

Keywords

OPC UA
OPC UA Configuration Tool
CIM-IO
OPC.Ua.DiscoveryServer