Material &1 in plant &2 for sales order &3 &4 with stock

What causes this issue?

You are trying to define a parallel unit of measure for a material or
to reset a definition, although sales order stock already exists for the
material.

System Response

You can only make this type of change if no stock exists for the
material.

How to fix this error?

Use transaction MMBE or /CWM/STOCK to check and clear existing stock.
You can then make the necessary change and repost the stock.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.