Here is a more complete explanation that I posted in another thread:
It's likely that the Record Type ID is not a valid record type for the particular object which is being synched. For example, if I assign a Lead record type to a Contact in MArketo, and then try to sync that, then the operation will fail because SFDC will not allow the Marketo to assign that record type to a contact. The solution is to clear invalid record type IDs when a Lead is converted, but still has a Lead Record Type ID.
Suppose I have a lead, and it has Record Type ID 'A'. Then I convert this lead into a contact, but the Marketo-side record still has Record Type ID 'A'. Since A is a record type for leads, the next time Marketo pushes an update to SFDC, it will also attempt to change the record type back to 'A' and this will cause SFDC to throw an error. This is the typical case where an invalid record type will be assigned. There are others but they are atypical and often unique.
There are many use cases for record types, so solutions are not universal, but one I recommend frequently is quite simple. Set up a trigger campaign like so:
Smart List
Trigger - 'Lead is Converted'
(Filter - Record Type ID is not empty) not necessarily required
Flow
Change Data Value - Attribute is Record Type ID
New Value is 'Null' or 'Valid Record type for Contact'
This will clear invalid lead record types upon conversion and allow you contacts to sync normally.