The DTD cannot be generated

What causes this issue?

The XML format for the IDoc types or extension selected is not
supported for SAP-Release 46 because the name &v1& does not correspond
to naming conventions for XML tag names. For this reason, a DTD also
cannot be generated.
Names that begin with a letter and have only a letter, number or
underscore as the following character are allowed. Names with a prefix
for the naming space are also allowed. The name &v1& does not meet
these requirements.

Procedure for System Administrators

Select the Unicode format. In the Unicode format, all special
characters are converted to XML according to the rules for displaying
ABAP data when creating tag names.

Error message extract from SAP system. Copyright SAP SE.