Incomplete data cannot be saved

What causes this issue?

The data entered is incomplete.
Mapping type 'Different Table Names': At least one table must be
assigned
Mapping type 'Different Tables/Field Names': At least all key fields
must be assigned
Mapping type 'Programmed Mapping': You must specify at least one source
and one target table, and a source or target function module. If
redistribution is active, you must specify a target function module.

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?

Complete the data.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.