Why does Marketo fail to add to SFDC campaigns

It might be a mapping problem. 

How were the fields created? Were they created in Salesforce?
Are the lead and contact objects mapped to each other so Marketo can read it properly?

In general, it shouldn't matter if it's a contact. This is likely related to the permissions that are setup, or potentially some validation rules you have in SFDC. Reach out to support? They are good at fixing this sort of thing.

Hi Abdallah,

What this error indicates is that when attempting to perform this operation, the sync user does not have the ability to edit either parent object of the Campaign Member, either the parent campaign or the parent lead/contact. Though this can be a result of missing Standard Object Permissions on Campaign, Lead or Contact, it is also frequently a result of an apex trigger firing during the insert, upsert, or update event on the Campaign Member object, which causes the operation to fail due to interfering updates or temporary changes at the time. Unfortunately due to the vagueness of the error message it is not possible for us to perform additional troubleshooting or provide additional detail about what may be causing the issue from the Marketo side. You will need to work with your SFDC admin in order to determine the cause of the issue.