Direct payment of tax required when no tax deducted or deduction fails; deductor may be deemed assessee in default. Section 391 requires the assessee to pay income tax directly where no provision exists for deduction at source or where tax has not been deducted as required. A special mechanism governs direct payment for specified securities or sweat equity from an eligible start up under section 289(3). If a person obliged to deduct tax (including a principal officer or employer) fails to deduct or remit tax and the assessee also fails to pay, that person is deemed an assessee in default under section 398(1).