You can use either the TurboTax online editions or the desktop CD/Download editions to complete and file a 2018 tax return.
TurboTax online editions for 2018 - https://turbotax.intuit.com/personal-taxes/online/
TurobTax desktop CD/Download editions for 2018 - https://turbotax.intuit.com/personal-taxes/cd-download/
For 2015 you will need to purchase, download and install on a personal computer one of the 2015 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2015 tax return can only be printed and mailed, it cannot be e-filed.
Note that if you are filing the 2015 tax return for a tax refund, the refund will not be paid. The time to file for a 2015 tax refund without an extension expired on April 15, 2019.
