After Samsung stops support, the community will likely offer custom firmware like LineageOS, but for now, stick to official Samsung builds.
You used the wrong CSC for your region, or you are trying to flash carrier firmware on an unlocked phone. Fix: Re-download the correct firmware. Use Frija and auto-detect your CSC by entering your IMEI. s21 fe firmware
In Odin, only "Auto Reboot" and "F. Reset Time" should be checked. DO NOT check "Re-Partition" or "NAND Erase". After Samsung stops support, the community will likely