This article covers 

Unable to convert a lead that is in use by workflow

[LeadConvertResult  accountId=‘null’  contactId=‘null’  errors=‘{1[Error  extendedErrorDetails=‘{0}’  fields=‘{0}’  message=‘Unable to convert lead that is in use by workflow.’  statusCode=‘RECORD_IN_USE_BY_WORKFLOW’ ] ,

Summary

When a Lead conversion is triggered by Chargebee during sync when the setting “Find and convert leads” setting is enabled. 

Reason for the error:

The error which resulted in the lead conversion failure is due to the 

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.

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

The above conditions will not allow the lead to be available for the conversion to happen. This is the reason Chargebee was unable to convert the lead when the sync happened.  

Related articles and Documentation

Integration guide for versions 1.37 and below

Integration guide for versions 1.38 and above