A REST API call on the Company API or Opportunity API fails with the error "1018: CRM Enabled -- The action could not be carried out, because the instance has a native CRM integration enabled."
Per the documentation on developers.marketo.com, the Company and Opportunity APIs are only available on instances that are not currently synced with a CRM such as Salesforce or MS Dynamics. If your instance is synced to a CRM, you will need to update Company and Opportunity information through the CRM rather than the API.
Documentation for these APIs can be found here:
Who This Solution Applies To
Marketo instances with a CRM sync