The Galaxy A20's bootloader has a bug where it refuses to load boot/recovery images that don't contain the SignerVer02 magic and an AVB Footer at the end of the partition with the image's size. This results in the "SECURE CHECK FAIL" screen that we all know and love.
You have two options:
- Flash the FixBootImage zip in TWRP. This will fix the boot image you have installed on your device.
- Use the Python script to fix the image & then flash it. Example:
python fix_bootable_image.py boot.img fixed.img