Hello there,
Basically when we try to add new task, it default the bucket value based on display order, and tries to match with older record which was deleted, so only option we left is to delete the To-Do Project bucket and create again.
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue.
https://community.dynamics.com/crm/b/friyanksblog/posts/project-operations-error-project-task-issue
Hope this resolves your Query !!
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