Boot image blocked by OEM lock on Samsung

I'm attempting to root a Samsung Galaxy A20s (Android 10). I've already patched the boot image with Magisk and enabled "OEM Unlocking" in Developer Options. I ran

adb reboot fastboot

However, when it reboots, fastboot devices shows no devices. Looking online, I found the odin4 utility and attempted to flash the phone by

tar cf boot.tar boot.img
./odin4 -b boot.tar

However, it failed with

Check file : boot.tar
/dev/bus/usb/001/120
/dev/bus/usb/001/120
Setup Connection
initializeConnection
Receive PIT info
success getpit
Upload binaries
boot.img
Close Connection
Fail request receive -5
FAIL close
Fail Close Connection

On my phone's screen, I see

OEM LOCK: ON (U)
...KG STATUS: CHECKING # I can't read the beginning of the line because it's in the round corner of the screen.
QUALCOMM SECUREBOOT: ENABLE (CSB)
AP SWREV: S1, T1, R1, A2, P2
AP SWREV: K 2, S 2
SECURE DOWNLOAD : ENABLE
DID : 
download mode !!
    Custom Binary(boot) Blocked by OEM LOCK

How do I unlock the OEM?