SharePoint Designer Workflow error - Coercion Failed: Input cannot be null for this coercion

This happens when the SharePoint Designer Workflow uses "Send An Email" action which refers to the email recipients form a list item site column.

This issue will occur when the site column containing recipients email-id/name is BLANK or NULL.

Make sure you handle the scenario when the list of email recipients is blank.

The best way to troubleshoot these kind of errors is to check the flow of your workflow step by step. Write logs at every stage to check at which point actually the issue is happening and work on that line once identified.