Differential item &1: No default value for subassignment &2

What causes this issue?

According to entry &V3& of currency translation method &V4&, the item
&V1& is intended for posting rounding differences.
The differential item has a required breakdown by the subassignment
&V2&. In the rounding rule, the "Default" indicator is selected for
this st subassignment, but no value was specified for the
subassignment. This is not permitted because subassignment &V2& does
not have a general default value.
This may have happened because the breakdown for item &V1& was changed
from optional to required, or because the "Default value" indicator was
deselected while maintaining the characteristic &V2&.

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?

Check the breakdown category settings of item &V1& with respect to
subassignment &V2& and the definition of characteristic &V2&. If
needed, change the entry &V3& of translation method &V4&.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.