This can happen with a very simple mistake where you enable the provisioning feature by setting up the Integration and saving it. But then forget to turn on the create, update, and deactivate users options. This error message lets you know that create users option is not on, as the error message is stating that it was unable to find a user in the Promapp application that matches this user, therefore it could not assign the application. If the Create user option is turned on, it would create a new user in Salesforce once it found no matching user existed, and assignment would succeed.
Link to Okta documentation