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.
You use "my" a lot in your question and at the end say "for myself and my husband." So it's not clear if you are referring to joint returns or if filing as "married filing separately."
In any case, a 2018 return can be prepared in either Online TurboTax (web version) or desktop software (CD/download), and can be efiled, if desired.
You wrote: "my 2015 that looks like it was not done."
What do you mean by "looks like" it was not done???" You have to know for a fact whether it was filed or not. Why did you word it that way saying "looks like"? Did you start one in Online TurboTax and never finished it?? Or could you have prepared and filed it in another account?
If you confirm that you did not file a 2015 return at all, you'd have to use 2015 desktop software (CD/download). It cannot be prepared online. And a 2015 return cannot be efiled. It has to be printed and mailed.
https://turbotax.intuit.com/personal-taxes/past-years-products/2015/
Also see the answer by DoninGA on this page about the deadline to claim a 2015 Federal refund.