We could sync with SharePoint tasks list from Project Professional, some default fields will be mapped, such as % Complete.
Then we could create a 2013 workflow based on the task list(Since the task list is a classic app, so Power Automate is not available) , if %Complete equals 100%, send email to users.
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More information for your reference:
https://learn.microsoft.com/en-us/office/troubleshoot/project/sync-with-tasks-from-project
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