Error "Create billing document" not allowed (Message no. VF032)

Hi gurus,

I need a little help for billing by t-code VF01:

I have created a Debit Memo Request and run VF01. When I enter the doc. number I got  "No billing documents were generated. See log"

Details are:

Diagnosis

    The status check for the status object VB0070000215000010

    indicated that the procedure "Create billing document"

    cannot be performed because User Status forbids it.

How can I change "User Status" ?

many thanks

emre