SAP SD - I can't change the exchange rate in the header of the Sales Order

Hi

I do have two sales order with the same type

In one SO, i can change the value of the exchange rate in

VBKD-KURSK

In another one, created few weeks after, the field is greyed-out and therefore i can't change it

What do i need to check to enable the change of this field ?

Thanks

stéphane