There is no license code needed for a 2018 tax return.
Find download using Windows - https://ttlc.intuit.com/community/installing/help/find-your-turbotax-for-windows-software-download/01/25572
Install TurboTax for Windows - https://ttlc.intuit.com/community/refund-status/help/how-do-i-install-the-turbotax-software-for-windows/01/25934
Find download using Mac - https://ttlc.intuit.com/community/installing/help/find-your-turbotax-for-mac-software-download/01/25584
Install TurboTax for Mac - https://ttlc.intuit.com/community/installing/help/how-do-i-install-the-turbotax-software-for-mac/01/26067
Troubleshoot install for Windows - https://ttlc.intuit.com/community/troubleshooting/help/troubleshoot-installing-and-running-turbotax-on-windows/01/25718
Troubleshoot install for Mac - https://ttlc.intuit.com/community/installing/help/turbotax-for-the-mac-installation-troubleshooter/01/25810
Does your computer meet the system requirements for the software?
There was no license code or number for 2018 desktop software. The license codes began with 2019 software.
Try contacting customer support for help with installing your software.
To call TurboTax customer support
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
Customer support is available from 5 a.m. to 9 p.m. Pacific time