As per the instructions. ( https://www.bungie.net/en/Help/Article/45831 )
Step 1- Sign-in to Bungie.net - Create or sign in to a Bungie.net account using Blizzard, PlayStation 4, or Xbox Live credentials.
Step 2- Redeem the Early Access Pre-order Code - Enter the Destiny 2 Early Access Beta Code provided by the retailer on Bungie.net/redeem.
Step 3- Retrieve the Destiny 2 Beta Download Code - When they are available, visit your redeemed codes page on Bungie.net and select PC.
Step 4- Redeem the Beta Download Code - Log into your Blizzard account to redeem your Destiny 2 Beta code in the Blizzard Shop.
You are currently on step 3 which requires you to visit https://www.bungie.net/en/Profile/Codes/
To generate your Pc code.
But Pc code generation is not yet available.
Check your codes page closer to the Pc beta.