Select valid variable; (set BP: first column of Editor)

What causes this issue?

In the classic ABAP Debugger, double-clicking a word in the source code
invokes the following actions:
Word = Variable -> Display variable in respective detailed view
Word <> Variable -> Sets a breakpoint in this row

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

In the new ABAP Debugger and the new ABAP Editor Control, this behavior
is different:
Double-clicking a variable displays it in the respective detailed view,
as before.
However, if you double-click a word <> variable, a breakpoint is no
longer created.
You can create or manipulate the breakpoint by single-clicking in the
lefthand column of the ABAP Editor Control, or through the context menu.
These changes have brought about new context-specific functions when you
double-click:
For example, double-clicking the start of a block (IF/METHOD/DO...)
automatically takes you to the end of the block.
(ENDIF/ENDMETHOD/ENDDO...)

Error message extract from SAP system. Copyright SAP SE.