Please check that the equivalent client in Megaventory has not been flagged as deleted.
If that is the case, presumably the client has been deleted directly in Megaventory which is not a recommended action given how the integration is supposed to work.
The simplest way to overcome this issue is to update the mapping in Magento database and assign the correct Megaventory ID to this customer. Feel free to contact us for directions on how to do this.