You cannot select the transaction currency. It is adopted automatically.

What causes this issue?

You marked the "Choose transaction currency" field.
This function cannot be performed because either
you entered an existing document as a reference
the "All currencies" indicator is not active in controlling area &V1&.

System Response

If you entered a reference document, the currency in the reference
document is transferred.
If the "All currencies" indicator is not active for controlling area
&V1&, you can only enter a document in controlling area currency. The
SAP System uses this currency automatically in the document.

How to fix this error?

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.