ERROR: unable to obtain exclusive access to this record

Cause

The error means that dataloader.io 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 edit.

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

You can find information of this error on Salesforce documentation:
https://help.salesforce.com/articleView?id=000351789&language=en_US&mode=1&type=1

Solution

You can find in the Salesforce article shared above some options to identify what is locking the record, anyway, the usual scenario is that the record will be available for exclusive access for dataloader.io task after some minutes.