You try to merge two records in Marketo, one SFDC Lead and one SFDC Contact, and the merge fails.
When you are trying to merge a Ccontact record with a Lead record and the Lead record is losing record, Salesforce will try to convert that Lead record to Contact and merge both records in SFDC and Marketo. If the Marketo Sync User in Salesforce does not have permission to convert the SFDC Lead to a Contact, the merge will fail in Salesforce and in Marketo.
To see if this is the problem, log into SFDC using the Marketo Sync User credentials and try to merge the records in SFDC. If you cannot merge the records while logged in as the Sync User this means that the Sync User does not have permissions in SFDC to covert a Lead to a Contact.
To resolve this ask your Salesforce Admin to change the permissions on the Marketo Sync User to allow it to convert leads.