You have set the Message Severity to "Error" in Fine Tuning > Materials > Message Severity Configuration to: Error.
You have created a Sales Quote ABC with the Product XYZ with the Unit of Measure as Each. When you changed the Quantity Conversion in the Product XYZ, you are not getting the error as per the configuration in fine tuning.
You can add a Quantity Conversion and delete it accordingly.
Reproducing the Issue
1. Go to the Products workcenter.
2. Go to the view Product Administration.
3. Open the Product XYZ.
4. Click edit and change the Unit of Measure.
5. Click Save.
Result: Your entries have been saved.
In this case for product XYZ, no quantity conversion is required because there is no other quantity maintained and this is the reason you are able to change/delete the quantity conversion without any error message.
Below is the scenario when the error message will be raised:
a) Missing quantity conversion : If there is any active sales data and sales Unit of Measure which is different from the product's base UoM, then the deletion of the quantity conversion would be restricted with Missing Quantity Conversion.
b) Quantity conversion change: If there is any active sales data and if the product is used in any sales document, then this error message will be raised while deleting the quantity conversion based on the fine tuning.
KBA , LOD-CRM-PRO , Products , How To