Support may know.
usually when a merge error occurs it is because Marketo cannot merge the records in the CRM.
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I actually did a write-up of this very issue for someone else a few days ago; I hope this helps you out.
Thanks Courtney! The duplicate was never added to Salesforce (dupe blocker) so I wanted to merge them to update the existing lead record with the history. I've done this before so I don't know what else is causing the error. I opened a case with support so stay tuned.