When you open a Form template using Easy Form Editor, the left side of the screen is blank due to which you are not able add any fields in the Form Template. But you are able to view the Form Preview (Sample Data).
Reproducing the Issue
1. Go to Application and user Management work center.
2. Select Form Template Maintenance view.
3. Open a Form Template with Easy Form Editor.
4. Where we can see the sample data preview but the right side of the screen is blank, hence you are not able to edit the form.
The reason for this behavior is, if the form template has been modified by the Adobe Lifecycle Designer then it cannot be modified by Easy Form Editor any longer.
This is expected system behavior.
KBA , AP-RC-BCT , Business Config. Tools (SAP Business ByDesign , Product Enhancement