Function code cannot be selected

What causes this issue?

You entered an inactive function code.
You can trigger this by:
inputting directly in the OK code field<(>,<)>
inputting the fastpath of an inactive menu function<(>,<)>
choosing an inactive function key, or even by
choosing ENTER, if ENTER has an inactive function.
In batch input, a function code is included in field <NP>BDC_OKCODE</>
of the session, as long as this function code is not assigned to ENTER.
As well as activating function codes statically in the Menu Painter
(Transaction SE41), they can also be deactivated dynamically using the
<NP>EXCLUDING</> addition with statement <NP>SET PF-STATUS</>.

System Response

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?

Error message extract from SAP system. Copyright SAP SE.