¿Why PO type FO is asking me to enter a contract while Creating the PO?

Hi all,

I am new for invoicing plans, so i read the manuals related, but when i try to do the PO type FO the system is asking me for  contract and don´t read anything about creating a contract before the PO so should i do? is there anything wrong with the PO type FO in my system?

Thanks

Jorge G