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The problem here is that the main branch of an automation rule always refers to the issues produced by the trigger.  So when the assign issue executes, it tries to assign the issue produced by the 'Scheduled' trigger.  That's just a scheduler though that doesn't add any issues to the rule and therefore the assign fails.

Step-by-step guide

To solve this problem you have to branch the rule using the 'Branch rule / Related issues' action for 'Created issues'

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Info

For the example above, you could also set the assignee directly in the 'Create issue' action simply by adding the field. Please ensure though that the assignee field is on the appropriate 'Create' screen in JIRA first.

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