Error in ME59N- Creation of Automatic PO

Dear Gurus,

I am getting error while creation automatic PO In ME59N. Below error shows

Error Name                                                                                          Massage Class          Msg No.

PO header data still faulty                                                                            MEPO                         2

Internal number assignment not defined (please enter number)                 06                              243 

I have done all the setting in Material Master & Vendor Master for Automatic PO Genration.

Aslo I maintain the source list & Info Record for two vendors.

I assign the source of supply in PR .

After this when I exicute in ME59N by entering Purchasing Group,Purchasing Org,Vendor,Plant

and selected the Per Requisition , per contract tab.

After this above two error is showing in SAP.

Kindly give us solution to resolve this.

Regards.

Abhijit