dataloader.io Record Lock Error During Data Operations

The record lock error, "unable to gain exclusive access to this record", means that dataloader.io from MuleSoft is trying to perform an operation over a record which is already being locked or edited by some other process (or person). Therefore, dataloader.io is not able to lock the record for editing.

Basically, there is some other process or person working with the same record, so Salesforce shows that error and stops the task to avoid data loss.

You can find more detailed information about this in Salesforce Error 'Unable to lock row - Record currently unavailable'.