When you receive the error code 403 'The request could not be authenticated or the authenticated user is not authorized to access the requested resource'   ( For a full list of errors please refer to ActiveCampaign API document here as the last screenshot).

Please check that you've set the account name and developer key correctly as per the screenshot below. 

You will also receive a 403 error when the developer key is changed, but has not been updated inside your subscription at  member.findmyinsight.biz. 

Once you've updated the account name and/or developer key, create a new data source to test the updated key or account name.

From the ActiveCampaign Website here