pls can you clarify, what did you buy from Costco isn't it the "download" edition rather than "online", which is a different service direct with turbotax and maybe where you are stuck on a loop on the website. You should be downloading the appropriate software from the link provided by Costco (if not see below) and there will be a screen to activate the software with the code, also some pages to sign into your TT account and register the product.
The download page is here https://turbotax.intuit.com/personal-taxes/cd-download/install-turbotax/