In SAP, the availability of certain fields like “Manufacturer Part Number” (MPN) in the purchasing tab of transaction MM02 can be dependent on various factors, including configuration settings and the material type being used.
The Manufacturer Part Number field might not be included in the screen layout of the purchasing view for certain material types. In SAP, you can configure different screen layouts for different material types using transaction OMS9. Check the field selection settings for the relevant material types to ensure that the Manufacturer Part Number field is included in the appropriate screen layout.