Don’t update your stock ROM anymore
I had the same issues. Please see THIS thread.
If you are in the USA and have updated the stock ROM to Android 10 (not sure about Android 9) I don’t believe you can get the RMM to normal. I have tried everything. This means the bootloader is unlocked but Knox/RMM blocks (edit: due to regional lock, non USA phone being used in USA) the TWRP flash to the bootloader. I had the same issue.
EDIT: If I could go back in time I would buy direct from the eStore or use different hardware (non Samsung).