If you are a Steam user that purchased the game before April 2019, Elite Dangerous will require you to input the keys you received at the time.
You can find them by right-clicking Elite dangerous in your Steam library and selecting 'View CD Key'. These keys can then be added at https://tools.elitedangerous.com/register.
Once on the page, please use your Frontier email address and password and enter the game keys in the 'Redeem Code' section of the page. You must do this once for each product key you have on your account.
If this does not work for you, or you do not have keys associated with your account, please submit a ticket to our Customer Support Team via the button below.