Same problem for me after I put in my key which i got from Humble Bundle, I think they might just not be released yet.
EDIT: I read a Mod's message from a different post saying
"That message is normal.
It shows the same for everyone.
You'll be able to generate your PC code closer to the beta."
Relating to "The Destiny 2 closed beta for consoles has concluded; it is open to everyone until July 23rd. Keys for downloading the PC Beta will be available in late August." message in Codes History on ur bungie profile
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Ahh that's good to know, they should really have made that more clear. I just spent a bit trying to figure it out.