@Lei we run into this same error when trying to update our employees' data via import from another system. We can't use internal ID and have to use an external ID. So I run an import of just mapping an external Id to each internal Id and then use that external Id when doing updates via another import.

I actually add a new field for this customer record, if I choose to add, it works, but the existing field can't be updated. If I choose to update only, it would ask for the internal ID/external ID