Drag And Drop is not working.

In this, I want to drag Value and drop into gray colour area after IF statement. It can not be drag into f1 field. While dragging into to gray field it fails to perform operation.

Inside gray area, it contains f1 field in which the element can’t be droppable, and I need to drop inside the gray colour area after the f1 field inside IF.

Please help me solve this problem.

Thank You.