Troubleshoot Your Process Using the Debug Log | Salesforce Developer Support

This video shows how to identify the root cause of failed process generated via process builder and what troubleshooting steps to follow in order to fix the errors.
Entity Key prefix decoder
Find Object type from Record ID prefix
Troubleshoot Processes
Process Builder Error 'The flow failed to access the value because it hasn't been set or assigned'
Error 'You don't have permission to do this' in Process Builder
Error 'Flow failed' when using Process Builder to access Contact fields on a Person Account