Validation/substitution/rule generation initiated

What causes this issue?

Part of the generated coding is missing. This can occur after copying
a client or after a transport.

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?

To initiate the code generation, press ENTER. Afterwards, you must
start the transaction again.

Error message extract from SAP system. Copyright SAP SE.