I had same issue, but here is how I fixed it.
First, make sure your device is on the list that allows for the beta version.
Second, download the 10.16 beta (Yes, the Mac Big Sur is using this name for beta version, not MacOS 11)
Third, after downloading the "Install macOS Beta", manually run the app and install it even you have upgraded to the 10.15.6 version.
Through the three steps, you should be able to upgrgade your Macbook to the latest MacOS 11 beta.
kevinelvellon
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution