Unit conversion is not possible from &2 to &3 for product &1

What causes this issue?

You are trying to convert unit of measure &v2& to unit of measure &v3&
for product &v1&. The conversion was not successful. There are several
possible reasons:
A product master does not exist for this product
There is no factor for this conversion in the product master
No characteristic has been specified for the product

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

To resolve the problem, you can do the following:
Check that product masters exist for all the products in the current
selection
Check that conversion factors have been maintained between unit &v2&
and &v3& for product &v1&
Check that characteristic <LB>9AMATNR</> has been included in the master
planning object structure or that another characteristic has been
assigned to the product.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.