I'm not able to download the El Capitan beta because
Code redemption temporarily unavailable?
I had a similar issue but it was related to user error but I think the Mac App Store app could have helped by asking me to check purchases. It was already downloading so I could not redeem a second time.
See https://forums.developer.apple.com/thread/9094 for details.
Hope this helps.
Thank you!
It worked for me too. I can't believe that there is so much amateurism in Apple.