Article 144 mandates that the Tax Authority must issue an official assessment for the taxes specified in Article 52 (Withholding Tax) if the tax was due but remains unpaid beyond the 14-day remittance deadline. The Authority can also make this assessment upon a specific application. The assessment notice will be issued in the name of the Omani payer (the person who made the payment), while clearly identifying the non-resident recipient from whom the tax was deductible. This establishes a formal legal debt for the unpaid withholding tax, allowing the Authority to initiate collection procedures.