When you work with InfoPath forms in the SharePoint environment and the form has a people picker control that accepts a person or a group column value, there is a chance that you may get the error "The form cannot be submitted. The user does not exist or is not unique".
Resolution
The error message suggests that this could be due to a permission issue of the user accessing the InfoPath form.
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Make sure that this list was not set to read-only. You need to check the InfoPath form as Page Design -> Properties and check the list settings -> Advanced Settings -> Item-level Permissions.
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Another alternate solution would be to remove the users using the site collection Admin and add the user and see if the error still exists.
Happy SharePointing :-)