shopping cart error

hai ,

i am using classic scenario.I have created shopping cart and tried to shop.after ordering it has been checked by check status and it is approved.when i go for follow on document to find out the purchase requisition number it shows error in process.Then i have checked in rz20 ,there i found out the below error.

shoppingcart 0000000031(preq 4000000002):06 355 pur.org 3000 not allowed for plant 555

can anyone please suggest me the correct solution

thanks and regards

n.chakradhar