Message type: E = Error
Message class:  FKK_ID_XX_ODN - ODN message 
Message number: 029
Message text: Number range assigned to the document class
You tried to change or delete the document class but the number range
has been already assigned to the document class in Customizing.
The system issues an error message and will not allow you to continue with this transaction until the error is resolved.
In the Customizing activity
<DS:SIMG._FICAOD_ODNNR_DET>Assign Number Range to Document Classes</>,
remove the current number range assignment to the document class. Only
then you will be able to change or delete the document class.
Error message extract from SAP system. Copyright SAP SE.
The SAP error message FKK_ID_XX_ODN029 typically indicates that there is an issue with the number range assignment for a specific document class in the SAP system. This error is often encountered in the context of contract accounts receivable and payable (FI-CA) when trying to create or process documents that require a unique identifier.
Cause: The error is usually caused by one of the following reasons: Number Range Not Defined: The number range for the specific document class has not been defined in the system. Number Range Not Assigned: The number range has been defined but is not assigned to the document class you are trying to use. Number Range Deactivated: The number range may be deactivated or not available for the current transaction. Authorization Issues: There may be authorization issues preventing access to the number range.
Solution: To resolve the error, you can follow these steps: Check Number Range Configuration: Go to the transaction code SNRO (Number Range Objects). Look for
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
FKK_ID_XX_ODN028 Number group assigned to the document class &1
What causes this issue?  You tried to change or delete the number group but the number group has been already assigned to the document class in Custom...
FKK_ID_XX_ODN027 Specify either function module or ABAP class if process type is selected
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FKK_ID_XX_ODN030 Number group not defined
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FKK_ID_XX_ODN031 No number range object assigned to country &1
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
Click on this link to search all SAP messages.