Different currencies in print document lines

What causes this issue?

When the payment scheme was adjusted for contract &V1& (within the
framework of interim and periodic billing), the system determined that
billing document line items exist with different transaction currencies.
This is not permitted for payment schemes.

System Response

Invoicing is terminated for this contract.

How to fix this error?

Make a note of the message, data and entries that lead to this error.
Create a problem message.

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.