Unknown account assignment (U) & Projects

I have created a Service Purchase Order using the account assignment U since the cost center will be different for each SES I will be doing and I dont know yet what are those cost centers. I have one SES to do and I need to charge it to a Project, but when I go and create SES the project field is as not available, is it my company's configuration? SAP standard allows what I'm trying to do? if not, what are my options?