Cost conditions in contract &1 can only have one currency.

What causes this issue?

The current error may occur in one of the following scenarios:
Incorrect configurations caused multiple currencies to be specified when
creating or modifying a contract.
You attempted to combine contracts with different currencies.
You attempted to reassign performance obligations to a contract that
uses a different currency.

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

System Response

The operation is terminated.

How to fix this error?

Make sure that conditions that belong to condition category "C" are
specified in the same currency within a contract.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.