Error translating the quantity to the CO-PA standard quantity

Hi FICO GURUs,

I am getting error when i am Post Sales Invoice to Accounts using transaction VF01 (Create Billing Document) and role(s) Customer Service Representative (responsible)

Error: _Error translating the qty to the COPA stand qty_

that the material master does not have a Statistic UoM maintained

Pls any one can help.

Regards