First you should ALWAYS save a PDF of the finished return AND the .taxfiles to a removable storage medium so you can access the return anytime. If you did not save the PDF then you may no longer be able to open the 2017 or 2018 .taxfiles because you may not be able to get updates or state programming for tax years that old.
Access your downloads here
https://shop.turbotax.intuit.com/commerce/account/secure/login.jsp?pagecontext=download
If you are an Advantage subscriber sign onto your Advantage account to download
And....there were no "license codes" for desktop software prior to 2019. Those began with the software for 2019.
If you used desktop software downloaded/installed from a CD to your own computer, then the tax files are stored locally on your own hard drive or any backup device you used like a flash drive. They are not stored online. If you cannot recover them due to a hard drive failure, etc. you can get a free transcript from the IRS or for a fee of $43, an actual copy of your tax return.