The previous workflow is stuck for some reason and a new one cannot be created until old one is either stopped or purged.
Navigate to Home > Administration > Settings > Property Settings.
Filter for 'etl'
Modify the three 3 returned settings to true:
Once done you will be able to see the ETL workflow which was stuck in the 'created' state in the Workflow list and purge it.
From then you will be able to run a new ETL process.