You can use the New Report button on the Report Builder home page to create a report from a web map. This web map needs to contain at least one feature layer.
Please note that your feature layer has to be either an ArcGIS Online hosted feature layer or an ArcGIS Server feature layer which is referenced in ArcGIS Online. Feature collections or any other type of ArcGIS Online layers will not be recognized by Report Builder. Please see the ArcGIS Online help for information on feature layer types.
It is important that all of the feature layer attribute fields that you wish to display in the report are selected in the Configure Attributes dialog in the web map (ArcGIS Online) and that their aliases are properly defined there. The Configure Attributes dialog is located within the layer options > Configure Pop-up panel in the web map. The aliases you define here will be used as the indicator names in your report.