When a layer is added to a map, the basemap is removed, and when attempting to add a basemap, it fails and returns the following error:
Error: The layer, '<Basemap name>', is not compatible with the current basemap. To add the layer, add it to the map as the basemap.
This is an expected behavior. Different basemaps use different spatial references. When a layer using a different spatial reference is opened in Map Viewer, the basemap is removed from the map. In the example above, the tile layer is not compatible with the World Topographic Map basemap from Living Atlas.
Use one of the following solutions to solve the issue: