Customer-defined fields for structure &1 have not been transferred

What causes this issue?

Customer-defined fields have not been written to the database for one
of the following reasons:
The key fields for the table parameters (EXTENSIONIN and EXTENSIONINX)
do not contain entries or do not agree.
The key fields for the table parameters do not agree with the key
fields for the corresponding import parameters (for example, PLANTDATA
and PLANTDATAX or in the case of the material number
HEADDATA-MATERIAL).

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?

Make sure that the key fields are the same for all relevant parameters.

Error message extract from SAP system. Copyright SAP SE.