Is your phone connected to the internet while trying to unlock? I think this is required for the unlock to work on Android 10.
You can try to obtain a adb logcat while trying to unlock, this should contain OemLockVerifier and OemUnlockPreferenceController output which might provide some details why it fails. (But make sure to remove your IMEI and Serial in case you want to post the output here)
Apart from that, there seem to be some issues with unlocking the bootloader, see also this thread.