Currently when you trigger an automation manually and you dont have a User created for that account the automation throws an error and you cannot run that automation again for that account even after you add a User.
A good solution would be the system letting you know that there is no user name on the account before running the automation.
Hi Alex, I'm on the Automations team at Vendasta, and I'm happy to let you know we are currently working to address this exact error handling issue!
With this release (coming around the end of February), you'll be able to resume an automation after it has errored. For the specific example you provided, if the user is missing you can action on this by adding a user, and then by going to the activity table you can restart the automation.
Let me know if you have any other questions!