Watchpoint cannot be created (area of validity varies)

What causes this issue?

The watchpoint variable and the variables used in the watchpoint
condition must have the same validity area.
For example, it is not possible to set a watchpoint on a global variable
(SY-SUBRC) and refer to a local variable in the condition.

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

How to fix this error?

Use variables with the same validity area for a watchpoint.

Error message extract from SAP system. Copyright SAP SE.