Error 'Unable to convert lead that is in use by workflow' on Lead conversion

To resolve this error, you need to identify and clear the automation that is currently holding the Lead record. The following sections walk through each possible cause.

This error appears if any of the following conditions are met:
 

1. There is a time-based workflow action pending that is related to the Lead. Or, there is a Process Builder with a time-based action pending that is related to the Lead.

• A pending Workflow action can be tracked down by going to Setup | Monitoring | Time-Based Workflow. Then filter on criteria related to the Lead record that encountered the error to see which workflow is implicated. Use "Contains" instead of "Equal" in the operation to get better results on search.
• Once you're able to verify the pending action, check the checkbox beside the pending action and click Delete.
• A pending Process Builder action can be tracked down by going to: 

2. The Lead that is being converted is currently in an Approval Process.

• This can be tracked down by adding the 'Approval History' related list to the page layout of the Lead record that is being converted and confirming whether there is a 'pending' entry in the 'Approval History' related list or none. 
• In the case of a 'pending' entry, recall the approval leads from the approval process. 
• Otherwise, perform the conversion after the approval process is completed.

3. This error can also be seen post-conversion, if a workflow gets triggered and one of its field update actions changes a field that is referenced in a validation rule.