Then you will need to factory reset the phone, and of course you did do a backup of your phone BEFORE you installed the beta, correct? And then you will do a restore of the backup using Smart Switch
Hello @FJM0 ,
As @user751201tmO21 and @useroVojrBY7ug have mentioned, its recommended to with draw from beta program before installing Android 11. How ever it is not needed to go back to Android 11. Only difference is you will still end up having to take beta software updates if you do not deregister from the program compared to a non beta user who would be able to upgrade directly from One UI 3 to One UI 4 in a single update.
Please let us know what error you are facing when trying to downgrade. Note that downgrade has to be done with smartswitch app on a pc and will not be over the air update. Also, an downgrade to Android 11 would completely erase your date on device, we recommend backing up all your data before attempting it.
One UI Beta Team