Add support for "Structures" layer types to be added as web services

[RyanBohan]

Please add support for the "Structures" layer type to be added as a web service in the map viewer.

Web Service:

https://carto.nationalmap.gov/arcgis/rest/services/structures/MapServer/20

[RussRoberts]

This isnt a structures service type its a Map Image Layer (Map Service) You cannot add a group layer in a Map Image Layer to Map Viewer. This is because when adding a layer from a Map Service URL like your sample

https://carto.nationalmap.gov/arcgis/rest/services/structures/MapServer/20

Goes through a different pipeline and makes a singular layer that falls under a Feature Layer. A group layer is not support on feature layers so the layer you are trying to add cannot be added. If you remove the /20 or add the individual layers in that group it will work as expected.

sub layer: https://carto.nationalmap.gov/arcgis/rest/services/structures/MapServer/21

[RyanBohan]

Gotcha, Thank you @RussRoberts 

You are correct and I got the group layer to add.  In this case I would suggest updating the error message to remove the "/20"