Referenced class or interface & not active/implemented

What causes this issue?

For data elements, structures, tables, and table types you can define
the data type properties in different ways. One option is to set the
type through a <DS:TX.DDDOC_REFTYPE>reference type</> - for example,
reference to a class or an interface.
Alternatively, the data type attributes for a data element can be
defined by specifying a domain or by direct type entry (direct
definition of the data type, number of places and if necessary the
decimal places). You can also define the type of the components of a
structure/table or of the row type of a table type by assigning a
<DS:TX.DDDOC_DDTYP>type</> defined in the Dictionary or with direct
type entry.
You defined a reference to a class/interface that is not active and/or
is not implemented.

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?

If you want to reference a class/interface, first activate and
implement this class/interface. Then activate it again.
Alternatively you can define the data type attributes in one of the
other ways described above and then activate it again.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.