Function module & does not exist yet

What causes this issue?

The function module defined for the classification matchcode does not
yet exist.
Your own matchcode records are not updated for classification
matchcodes (update type K). The application-specific search path is
allocated to the matchcode ID using a function module. This must be
defined when defining the matchcode ID.

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?

Check your entry and create the specified function module if necessary.

Error message extract from SAP system. Copyright SAP SE.