To create a report using the InstantAtlas Report Builder you need a web map from ArcGIS Online as a basis. This web map needs to contain at least one feature service. If it contains more than one feature service, you will be asked to choose one of them when you create a new report. The web map needs to be saved within your organisational ArcGIS Online account.
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 detailed information on feature layer types.
It is important that all the feature service attributes fields 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.