"INCOMPATIBLE DISK, uses features which are not supported on version of macOS" on startup.

Did you erase the whole physical drive or just the "Macintosh HD" volume?

https://support.apple.com/en-us/HT208496

Otherwise you will need to post your question on the Big Sur Beta Forums as it may be a side effect of the Beta OS you used. You are not allowed to discuss any details of Big Sur here on the regular forums as noted in the NDA you agreed to when downloading & installing Big Sur. This is one of the risks of using Beta software.

Hi Nick,

Maybe this can be the solution to the problem. I had the same problem completely. I returned from macOS Big Sur Beta to Catalina and I had the same problem. I searched for a solution, but when I looked at it, I couldn't find much resources. I talked to Apple but we couldn't reach a clear result. I got the solution like this: I was back in Catalina but got the "Incompatible Disk" warning, upgraded back to Big Sur and started "Internet Recovery". Here there are disks with strange names in "Disk Utility" on the left: for example mine is "disk1s5". Its total size is 0 KB and it could not be deleted, when I started "Internet Recovery" in Catalina. In Big Sur I was able to erase and reset the disc. When I erased this strange named "disk1s5" disk and installed Catalina again, I did not receive any "Incompatible Disk" warning again.

I had the same problem. I only deleted the volume from the disk (Macintosh HD), not the entire disk (APPLE SSD) and the same error appeared after I reinstalled macOS Catalina (before I had macOS Big Sur).

I tried to erase the entire disk (APPLE SSD), APFS format and GUID partitioning. I then reinstalled macOS Catalina and the error did not appear.

I used the steps from Apple in the link to delete the entire disk.

https://support.apple.com/en-us/HT208496#erasevolume - "How to erase your disk"

Hello, my English is not very good. I will try to explain my solution. I made a startup disk with the Beta Big Sur version, without deleting Catalina. Start my mac with that startup disk and 
then in disk utility I could erase that partition that brings us so many problems, then restart 
the mac normally and voila, problem solved.

Hello, my English is not very good. I will try to explain my solution. 
I made a Boot disk with the Beta Big Sur version, without deleting Catalina, then I Start my mac with that boot disk and then in disk utility I could erase that partition that brings us so many problems, then restart the mac normally and voila, problem solved.

As so many of us have this issue it is a 'feature' of the Big Sir upgrade. I can only move from Mojave to Big Sur via an 'option' start and then it is calling Big Sur 'OSEFI'. Startup Disk does not see the BS partition in Mojave. I shan't be too fussed at this stage - I never liked Catalina - even the pic of it looked like a Crocodile! So I'll play with BS but work with Mojave.

You would be surprised at how many people don't realize the risks of using Beta software. They think it is just a free preview that will just work without issue.

Good luck.