ArcGIS Enterprise 11.0: How to delete the topographic service from the web map viewer?
You can't remove the last remaining basemap layer. It sets up a lot of the Web Map, including the coordinate system.
What you can do is:
To customize the basemap you will need to add another layer (e.g. Esri world imagery) into the basemap before you can remove the topographic layer.
https://doc.arcgis.com/en/arcgis-online/reference/choose-basemap.htm
Jamal,
the Topographic service is your basemap. Every web map has a basemap. You can use the Basemap tool to select another basemap that will be less distracting. The Light Gray Canvas would be a nice choice.
Another option is to use one of your own layers as the basemap but I don't think this is possible with a hosted feature layer. I know when I add a map service from our own ArcGIS Server I have the option to use it as the basemap:
Bernie.