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

SAP Error Message - Details

• 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.