The DTD cannot be generated

What causes this issue?

No tag name could be created for the &v1& name, the DTD cannot be
generated.

System Response

The XML format is not supported for the IDoc types or extension
selected.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.