Section 15A defines Deductions in computing income chargeable under the head “Income from Property (usually rental income). These deductions are allowed while calculating final tax on rental income under Section 15.
Section 155 defines, the person paying the rent will deduct WHT while paying rental income. No deductions are allowed as listed in Section 15A while calculating WHT under section 155.
Both are explained in our article Rental Income Taxes which you can read if needed.