Material number is initial, construction type is &1

What causes this issue?

Synchronous maintenance of material number and construction type are
intended in Customizing, or to be precise: If the material number is
maintained, then the construction type must have the same value.

System Response

If the material number of the equipment is initial, the construction
type can be maintained however you like. However, a later maintenance
of the material number leads to the value of the construction type
being automatically set the same as the material number.

How to fix this error?

Changes to synchronously maintained values are only possible using the
material number. If you do not want this synchronous data storage, you
should either change the type of the equipment or change the
Customizing settings.

Error message extract from SAP system. Copyright SAP SE.