Select only one dynpro element

What causes this issue?

You attempted to convert the selected area to a box, but it contains
more than one element.

System Response

Unable to convert to a box.

How to fix this error?

Reduce the size of the selected area with <LS>Select block end</> or
choose <LS>Back</>, <LS>Graphical element</> and <LS>Convert to box</>.

Error message extract from SAP system. Copyright SAP SE.