Where is my mortgage interest deduction on my tax return?

The requirements are that you own the real estate, that the loan is secured by the property, and that it is qualified debt.

See IRS Pub 936 https://www.irs.gov/publications/p936/ar02.html#en_US_2016_publink1000229894
which says in part:

Secured Debt

    You can deduct your home mortgage interest only if your mortgage is a secured debt. A secured debt is one in which you sign an instrument (such as a mortgage, deed of trust, or land contract) that:

        Makes your ownership in a qualified home security for payment of the debt,

        Provides, in case of default, that your home could satisfy the debt, and

        Is recorded or is otherwise perfected under any state or local law that applies.

    In other words, your mortgage is a secured debt if you put your home up as collateral to protect the interests of the lender. If you cannot pay the debt, your home can then serve as payment to the lender to satisfy (pay) the debt. In this publication, mortgage will refer to secured debt.