How To Fix DT221 - Field & does not lie within customer namespace

DT221 - Overview

• Message type: E = Error

• Message class: DT - Table activation (incl. indexes)

• Message number: 221

• Message text: Field & does not lie within customer namespace

What causes this issue?

Fields of <DS:TX.KUNDEN-INCLUDE>customizing includes</> and
<DS:TX.DDDOK_APPEND_STRUKTUR>append structures</> should lie in the
customer namerange. You can then be sure that there will be no
conflicts with SAP field names in future Release upgrades.
The customer namerange for fields is YY and ZZ.

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?

If they are new fields or fields which are only used in structures, you
are recommended to change the field names and the reports which access
them.
The field name for fields of database tables cannot be changed without
the corresponding data being lost. It is only necessary to change the
field name if the same name is used by SAP.

Error message extract from SAP system. Copyright SAP SE.

DT221 - Details

• The SAP error message DT221, which states "Field & does not lie within customer namespace," typically occurs when you are trying to use a field that is not defined in the customer namespace. In SAP, the customer namespace is reserved for custom developments, and fields or objects that are not defined in this namespace cannot be modified or extended by customers.
  
  Cause:
  
  Field Definition: The field you are trying to use is part of the standard SAP namespace and is not intended for customer modifications.
  Namespace Confusion: You may be trying to access or modify a field that is not defined in the customer namespace, leading to this error.
  Development Objects: If you are working on a custom development (like a Z-program or Z-table), and you reference a standard field incorrectly, this error can occur.
  
  
  Solution:
  
  Check Field Definition: Verify the field you are trying to use. Ensure that it is either a custom field (defined