In order to export data, your Ninja Forms must have an active Store Submission action which saves data to your WordPress database.
EXPORT FORM SUBMISSIONS
Navigate to Ninja Forms > Excel Export
After navigating to Excel Export you will see the following:

- Select one of your forms via the Select a form drop-down list.
- Filter Submissions – You can filter by date or by the data included in specific fields
- Under Select fields, choose what fields you wish to export.
- Choose the File Type to be exported.
- Click the Download Excel file button.