Yes, you can amend your 2018 tax return to claim the mortgage insurance deduction. In fact, if you entered the mortgage insurance premiums when you prepared your 2018 tax return, TurboTax may automatically start your amended tax return with that change.
Once your adjusted gross income (AGI) reaches $100,000 ($50,000 married filing separately) the deduction for mortgage insurance premiums starts to "phase out". Once your AGI exceeds $109,000 ($54,500 MFS) you cannot deduct your mortgage insurance premiums.
Click this link for instructions on how to file an amended return with TurboTax.