When running the Enrich Layer tool in ArcGIS Online, the layer is not listed as input, or the Enrich Layer tool is not available as an option when performing analysis on the layer.
The layer is not a feature layer. The Enrich Layer tool requires access to feature layers and attribute tables to run the analysis and write the results. The Enrich Layer tool does not support non-vector layers such as map image layers and tile layers.
Use a vector version of the layer and publish the layer as a feature layer. Alternatively, run the Enrich Layer tool on a feature service.