Edit: You can follow the solution of @ashyx through the following link to fix the problem can not flash twrp or boot via Odin. I think it will work 
Link: https://forum.xda-developers.com/sa...g-sm-a520f-2017-kernel-source-t3557622/page31
Hello everyone,
After I updated the latest firmware, I was unable to flash BOOT or TWRP via Odin. It just returned the result that ONLY OFFICIAL BINARIES ARE ALLOWED TO BE FLASHED (RECOVERY / BOOT). I think samsung locked the bootloader. Today, I have found a way to fix this problem because knox 0x1 but can not root is not pleasant at all!
I only tested on my A520F, not tested on any other device. I am not responsible for any errors on your device.
1. Go to http://updato.com and download 2 firmware updates with your CSC.
+ Latest firmware 7.0 (November security).
+ Latest firmware 6.0 (MM).
2. Backup your important data
3. Reboot to download mode.
4. Add AP, CP, CSC of firmware 7.0 (November security) to the corresponding slot in Odin.
5. Add BL file of lastest firmware 6.0 (MM) to BL slot in Odin.
6. Start and wait a moment.
7. Setup and after that, you can flash TWRP or BOOT via Odin without bootloader errors.
Enjoy!
Sorry for my english...
Link: https://forum.xda-developers.com/sa...g-sm-a520f-2017-kernel-source-t3557622/page31
Hello everyone,
After I updated the latest firmware, I was unable to flash BOOT or TWRP via Odin. It just returned the result that ONLY OFFICIAL BINARIES ARE ALLOWED TO BE FLASHED (RECOVERY / BOOT). I think samsung locked the bootloader. Today, I have found a way to fix this problem because knox 0x1 but can not root is not pleasant at all!
I only tested on my A520F, not tested on any other device. I am not responsible for any errors on your device.
1. Go to http://updato.com and download 2 firmware updates with your CSC.
+ Latest firmware 7.0 (November security).
+ Latest firmware 6.0 (MM).
2. Backup your important data
3. Reboot to download mode.
4. Add AP, CP, CSC of firmware 7.0 (November security) to the corresponding slot in Odin.
5. Add BL file of lastest firmware 6.0 (MM) to BL slot in Odin.
6. Start and wait a moment.
7. Setup and after that, you can flash TWRP or BOOT via Odin without bootloader errors.
Enjoy!
Sorry for my english...
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