Start by enabling Enhanced Logging for Zapier
Go to Manage --> Business Settings --> Integrations and click on your Zapier integration, then select Enable Enhanced Logging.
Once you have it enabled, try again to trigger the action that was failing, then return to your Zapier integration and click View Logs.
Using the logs, you can determine what is causing your Zap to fail, or chat with Support and send them the logs so they can help you determine how to resolve the error.
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