Hey I have been getting this error for months and I have spoken with both SFDC & Marketo on different calls but no one was really able to give me a clear answer and I haven't been able to circle back with them (busy busy!).
It would appear the DupeCatcher app is preventing the Marketo Sync user from creating new leads.
I would suggest contacting support for that app and see if there's a way to exclude the Sync user from DupeCatcher validation, since apps like that are really intended for user input screens not automated de-duping.
If what Justin recommends does not work, search in salesforce another lead/contact that is likely to be a duplicate with the one you are trying to insert. 2 outcomes here :
-Greg