Transformation to field control only possible for single element

You have chosen the 'Convert to field control' function. However, the
area selected contains more than one element.

System Response

You cannot convert to a field element.

How to fix this error?

Reduce the area selected using the 'Mark end of block' function or
choose the 'Back', 'Graph. element' and 'Convert to field control'
consecutively.

Error message extract from SAP system. Copyright SAP SE.