Cannot create billing document from sales document

Hi,

i am trying to create billing document from the created sales document.

But the billing document is not generated. I was getting  the error - ' Document is blocked from billing'.

SO, i changed the billing block and left it blank, and when  I tried to create the document again, I got another error " Create billing document" not allowed (uSER STATUS   ST01, Object VB006000071200000).

Please help me out,