Asynchronous serial-/equipment numbers exist

&CAUSE&,,
Serial numbers, which are not synchronous with the equipment number,
already exist for this number.

System Response

Therefore, the serialization level cannot be set to <LS>Keep equipment
number and serial number synchronous</>.

How to fix this error?

Make sure that the serial numbers and equipment numbers are
synchronous.
Choose the desired serialization level.

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.