How To Fix U9504 - Changed entry in table with new entry in "Assignments" tab not allowed

U9504 - Overview

• Message type: E = Error

• Message class: U9 - Message Class for Output in Application Log

• Message number: 504

• Message text: Changed entry in table with new entry in "Assignments" tab not allowed

What causes this issue?

An entry to be changed or deleted in the following table contains
routing in its key that is to be entered in the plan assignment table
PLANMAP. This is not allowed.
Table / specific table key: &1 &2 &3 &4
Explanation of specific table key: Plan number

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?

Error message extract from SAP system. Copyright SAP SE.

U9504 - Details

• The SAP error message U9504, which states "Changed entry in table with new entry in 'Assignments' tab not allowed," typically occurs when there is an attempt to modify an existing entry in a table that is not permitted due to the system's data integrity rules. This error is often related to the assignment of objects or entities in SAP, such as cost centers, internal orders, or other master data.
  
  Cause:
  
  Data Integrity Rules: The system enforces certain rules to maintain data integrity. If you are trying to change an existing entry in a way that conflicts with these rules, the error will be triggered.
  Incorrect Assignment: You may be trying to assign a new entry to a field that is already populated with an existing entry, which is not allowed.
  Authorization Issues: Sometimes, the user may not have the necessary authorizations to make changes to the entries in the specified table.
  
  Configuration Issues: There may