DDL source & of type & cannot be converted to & (-> long text)

What causes this issue?

A DDL source defines an SQL view, a table function, an enhancement for
SQL views, or a table entity. A conversion with these DDL source types
is currently not possible, just like the conversion to another ABAP
Dictionary type -- for example, a data element or a table.
Neither is it possible to delete a DDL source of a DDL source type and
afterwards to create it again with a different DDL source type and keep
the name.

System Response

Check or activation was not successful.

How to fix this error?

If you want to create a DDL source with a different DDL source type,
delete the current DDL source and create a new one. All the newly
defined artifacts are to get new names in this process.
If you need to create another dictionary type with the entity name of
the DDL source, the DDL source in this case must also be deleted. Only
then can you create the dictionary type.

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.