@DaveF1006 I’m not clear that this is “investment interest.” It appears to be interest on a rental property.
Generally speaking, investment interest, such as money you borrowed to buy stocks, would be a miscellaneous itemized deduction subject to the 2% rule. That deduction was eliminated in 2018 for the federal tax return, although you can still list it in TurboTax because it may be deductible in your state.
However, interest incurred in borrowing money to purchase a rental property is probably deductible on schedule E, even if it is not a secured mortgage, as long as the tracing rules can be applied. The borrower can certainly claim interest that the borrower paid even if the borrower does not receive a 1098 form. The lack of a 1098 may trigger an IRS letter, but if the borrower can prove the interest paid by showing loan documents, then the burden falls on the lender for not issuing the 1098.
