Error when converting currency

What causes this issue?

An error occurred during conversion of the currency amount.
The following are possible causes:
The currency fields in the data records on which the method is based
have not been completed or have been completed incorrectly.
The system does not recognize the currencies that are specified in the
data records on which the method is based.

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?

Note that returned currency amount fields can contain errors or be
empty.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.