Assignment of single object to cost object &v1& not allowed

What causes this issue?

<DS:GLOS.cost_object_ID>Cost object ID</> &v1& has the
<DS:GLOS.cost_object_category>cost object category</> &v2& for which
the assignment of the single object is set.

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?

Do one of the following:
Assign a different single object to cost object &v1
Assign the single object to a cost object with a cost object category
that allows such an assignment
Change the setting for cost object category &v2& so that the assignment
of the single object is allowed

Error message extract from SAP system. Copyright SAP SE.