Most Likely Causes:

    The product template shows Units, but a specific variant may have a different UoM set.

    The Sales Order line UoM field is editable on the SO. Someone may have changed it directly on the line to MT without realizing it conflicts with the product's category.

    If the Sale order was created via import (CSV/Excel) or an API/EDI integration, the product_uom field on the line may have been set to MT programmatically, bypassing the UI validation.

    Someone may have moved MT (Metric Ton) into a non-weight category in your UoM configuration, or Units may have been moved out of their default category.

Hello,
I hope you are doing well

Fix:
1. Go to the product → check its Unit of Measure
2. Go to the sales order line → ensure its UOM is in the same category
3. If mismatched, correct the SO line UOM or re-add the product

I hope this information helps you

Thanks & Regards,
Kunjan Patel