You already posted about this a few days ago.
If you have redeemed your download code, then you have the Sierra beta in your App Store/Purchased tab.
The name is "macOS Sierra Public Beta", do you see that?
Click the Download button. The download, if it has already started, may not be apparent to you, until the installer app launches after the nearly-5GB download.
If the button says "redeemed", and not "Download", then go to the Store menu in App Store, and click on Check for Unfinished Downloads.
If you get nothing as a result, then Sign Out of the App Store (same Store menu), Quit App Store. Restart your Mac. Launch the App Store, log back in, and check the status button in your Purchased tab.
If you have NEVER downloaded the Public Beta, and that button only says Redeemed, then contact Apple for some resolution on this. It's likely an App Store problem that AppleID support is probably the best choice to fix.