Has anyone else had this issue? I'm encountering the same issue and can't seem to find where this field is located.
I just spoke with Salesforce support and you need to create a custom field and get the AccountId field value. The AccountId field is not visible in the opportunity that is why you need to create a custom field. I was able to achieve this by having a call with Salesforce support and they walked me through how to do it.
However, although it is now displaying the AccountId field in the opportunity object and Marketo has both read and write access to the field, i'm still getting that error and cannot figure out what is going on. Does anyone have any insight into what to do at this point?