I am trying to run this sql command and it keeps coming up ORA-02270: no matching unique or primary key for this column-list

create table Purchase_Order(P_OrderNo number PRIMARY KEY,P_NameofRetailer varchar(20),P_VendorID number references vendor(V_VendorID),P_VendorAddress varchar(20) references vendor(V_Address),P_Quantity number, P_UnitPrice number,P_TotalAmount number);

I am not seeing where to add the matching unique or primary key for this column?