I forgot to use my 2018 1098-T tax form for my 2019 tax filing so can I use it this year for my 2020 tax filing?
No, you cannot enter a 2018 form 1098-T on your 2019 tax return.
If you omitted your form 1098-T in 2018, you'll need to amend your 2018 tax return.
To amend your 2018 tax return, please follow the instructions in this TurboTax Help article.