doc. curr. field (VBAK-WAERK) is greyed out in sales order

Hi

Doc. Curr. field (VBAK-WAERK) in sales order Header-Sales tab is greyed out, even when creating new orders. I could not find the relevant configuration where you can make the Doc. Curr. field modifiable.

Is this some configuration issue or may be because of some ABAP progarmming? Please help.

Thanks in advance

Koustav