I can not create new Purchase Orde

Hi experts,

I have a problem, i am tryin to create a new Purchase Order by the portal, i go to tab Purchasing, and after, i click in Purchase Order. Appear a new windows, where i have to select the system:

- Local SRM System where appear different purchase order

- R3 US Back end where not appear nothing.

If i select one of the Local SRM System, and click in create button, appear an error:

Purchasing group could not be determined; inform system administrator.

What happend?

Thanks in advance,

Regards,