Material &1 in plant &2 with stock, check using transaction MMBE

What causes this issue?

You are attempting to define a parallel unit of measure for a material
or to reset the definition, although stocks already exist for the
material.

System Response

You cannot make this kind of change if the material is without stock.

How to fix this error?

Using transaction MMBE, check which stocks exist (plant, storage
location, batch, special stock, ..., historical stocks). If necessary,
book these out. Afterwards, you can make the required change, and book
the stock in again as appropriate.

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.