Hi. I need to file my 2015 and 2018 tax returns. Can I do that on this site? If yes, can you tell me what I need to do. Thanks!

No, only current year (2019) tax returns can be filed electronically using TurboTax Online.

There are versions of TurboTax available for you to purchase and download in order to prepare the 2018 return.  You must get the 2015 tax forms directly from the IRS.  Here is a link:  IRS Forms, Instructions, Publications

See the following TurboTax article for more information and instructions:  How do I prepare a prior year tax return?

Be aware that if you are filing your 2015 return to claim a refund, it is too late and you will not receive the refund.  If your 2015 return shows a balance due, then you are still required to file and pay the balance due.

@salston1123