You can use the New Report button in the Report Builder Editor top menu to create a report from a web map. This web map needs to contain at least one feature layer. If it contains more than one feature layer, you will be asked to choose one of them when you create a new report.
Please note that your feature layer has to be either a hosted feature layer or an ArcGIS Server feature layer. 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 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.