TurboTax 2018 didn't use an activation code. Please explain the context of your question. It's not clear what you mean.
What do you mean by "existing license?" Are you referring to a previously-purchased 2018 tax product? If so, did you buy 2018 TurboTax directly from TurboTax, other retailer, etc.?
Do you have the 2018 CD or downloaded installation file?
