Hello, I spent a lot of money on dental in 2016-2017 and was wondering if I can claim that on this years tax return.

No, you cannot include money you paid for dental in 2016 and 2017 on your 2018 Tax Return, you can only deduct expenses that were actually paid in 2018.

However, you can Amend your 2016 & 2017 Tax Returns to include the dental expenses paid in those years. You can click here for instructions on Amending your tax return: How to Change a Tax Return You Already Filed.