All S21 devices that participated in beta program should have official release available. If roll out is still in progress for specific carrier , You may reach out to carrier to check on their schedule.
Although inconvenient, Best option at this time would probably be to deregister from the program and downgrade to Android 11 if you still have an issue with updating to Android 12.
1. To withdraw from the Beta program, you must apply for withdrawal through Samsung Members application on your device.
("Settings > One UI Beta Program status > Withdraw" menu)
2. To remove the beta software, perform a rollback to the previous release version, using the Smart Switch PC Version on your computer. The rollback resets the device and removes data generated during beta testing.
Note: If you backed up your device data to your computer with Smart Switch, you can restore the data after the rollback process. To perform the rollback, connect the device to your computer with a USB cable. On the computer, launch the Smart Switch application, and select "Downgrade".
If you need assistance having your software rolled back to the latest approved version you can call us at 1-800-726-7864 and request the location to the nearest Service Center.
One UI Beta Team