I’m having this error, but I set correctly the External Id, just in Zapier been matching based on the real Account Id in Salesforce (I tested with static value and test past). What else I need complete to tell Zapier or Salesforce please compare with Custom External Id.
Failed to create a record in Salesforce
Could not update the Account record with ID R-PJKL9eGyWdrNzaRuvw8hhJ: Provided external ID field does not exist or is not accessible: R-PJKL9eGyWdrNzaRuvw8hhJ