You have 3 years from the due date of the tax return or file for a tax refund. For tax year 2018 you would need to file on or before April 15, 2022.
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2018 tax return can only be printed and mailed, it cannot be e-filed.