Customer field is grey out in VA41

Hi,

I am in the midst of configuring a new contract type in SD and I found out that after I have key in all these fields, Contract Type, Sales organization, Distribution channel,Division,Sales office & Sales group on the first screen of VA41, I cant key in the customer no in the second screen. The field is grey out. I need this field to input the customer no. Any kind soul out there can tell me how to solve this problem?

Thanks!