Watchpoint cannot be created (area of validity)

What causes this issue?

The watchpoint variable, or a variable of the condition, has a validity
area that is not supported.
For example, it is not possible to set a watchpoint on a row of an
internal table. (ITAB[1]-COMP or ITAB[1])

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

Error message extract from SAP system. Copyright SAP SE.