The merge process is encountering a Salesforce error while initiating the merge. To get the details of which error Salesforce is returning you'll need to file a Marketo support ticket.
If you were merging a Marketo person with no SFDC type, then you can merge it with a SFDC lead or SFDC contact no problem. However, since you are attempting to merge an SFDC lead with an SFDC contact, this will not work as they are not like objects.
I would recommend manually converting the SFDC lead to the same account as the SFDC contact and then merging the two SFDC contacts.