After deleting a lead or contact in Salesforce that has been synced to Marketo as a person record, attempting to merge that synced Person record with another Person record in Marketo keeps failing.
When a merge is performed in Marketo, the Marketo Sync User will first automatically attempt to merge the associated records in Salesforce. If the records have been deleted in Salesforce this attempt to merge the records in Salesforce will fail, meaning that the rest of the merge operation will fail in Marketo.
To complete this merge, the best option is to perform the merge through the API. The Merge Leads endpoint allows you to skip the automatic Salesforce merge attempt that will always run when performing the merge through the Marketo UI. Marketo support does not troubleshoot or write custom API calls like this, so it is best to work with your in-house developer or IT staff to create these API calls.