Exchange rate and currency adopted from document &1

What causes this issue?

Document &V1& comes from SAP Transportation Management (SAP TM).
A fixed exchange rate has been defined in the entered TM document &V1&.
For this reason, you can only use the currency and exchange rate agreed
in the TM document when posting.

System Response

You can no longer change the exchange rate.

How to fix this error?

If you get this message after assigning a TM document, you cannot select
this TM document for this invoice document. You must cancel the
selection and process the invoice item for this TM document separately
in its own invoice document.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.