You don't do anything. If you don't have a mortgage, you have nothing to enter in your tax return about the home. Just skip the mortgage interest topic. If TurboTax asks if you made payments on a home loan, answer No.
If the mobile home is on land that you own, and you pay real estate tax for the land, you can enter the real estate tax as an itemized deduction.
or do you mean you did take out a mortgage, but forgot to claim it on your tax return?
Or....did you pay sales tax for the mobile home? IF you have enough itemized deductions to exceed your standard deduction (and many people do not have enough) you can enter the sales tax you paid for the mobile home as an itemized deduction.
SALES TAX
You can enter the sales tax you paid for the mobile home you purchased in 2021 by going to Federal>Deductions and Credits>Estimates and Other Taxes Paid> Sales Tax. You will be asked if you paid sales tax on a major purchase, and you will be able to enter the sales tax you paid for your new vehicle.
Sales tax is an itemized deduction. “Major purchases” that you can enter for the sales tax deduction include:
Motor Vehicles (cars, trucks, motor homes, RV’s, sport utility vehicles and off-road vehicles
Aircraft or boats
Mobile homes
Manufactured housing
Building materials for major home improvements
You cannot deduct: furniture, jewelry, home electronics such as TV’s or computers
https://ttlc.intuit.com/questions/2566624-how-much-sales-tax-did-i-pay-last-year
https://ttlc.intuit.com/questions/1900791-how-do-i-find-my-local-sales-tax
https://ttlc.intuit.com/questions/2566624-how-much-sales-tax-did-i-pay-last-year