ArcMap 10.6.1 is unable to access a custom locator service if the service is not from a federated server and does not require credentials, is configured for geocoding with Portal for ArcGIS 10.6.1.
Secure the locator service so that it requires credentials.
Add the custom locator service as an item in the portal and store the credentials.
Perform the workflow as described in the following documentation: http://enterprise.arcgis.com/en/portal/latest/administer/windows/configure-services.htm#ESRI_SECTION2_CD97431BAC104D9A80DEA0D55F4420D9