No conversion found for SET to NOS-Message no. 8I171

Dear All,

At the time of Import PO creation with currency USD. system issues the following meesage

'No conversion found for SET to NOS

Message no. 8I171

Diagnosis

The UOM conversion does not exist

Procedure

The UOM conversion does not exist in the material master for the material and UOM entered'

I have varified Material Master,Info record etc but there is no different unit of measures for the material.

We are using Numbers as BUM and Order unit.

Please give the solution

Thnks