According to the others is -w option is somehow broken and should be replaced with fastboot erase <partition>, e.g.:
fastboot oem unlock
fastboot erase boot
fastboot erase cache
fastboot erase recovery
fastboot erase system
fastboot erase userdata
fastboot flash bootloader bootloader-angler-angler-03.68.img
Flashing the correct img solved the problem:
heimdall flash --RECOVERY TWRP-188.8.131.52_A20_V1.2_by_Agrim720\[Black-themed\].img.img
File was found here: XDA-forum
Previously rooted with boot.img patched by magisk and vbmeta flashed:
heimdall flash --BOOT magisk_patched.img
heimdall flash --VBMETA vbmeta.img
I highly recommend to ...
I refer to the Developers read me file for linux command line instructions:
Flashing Firmware from Command Line:
1. Fully charge your phone (use the wall charger as it's faster).
2. Download a decrypted device ROM or a Heimdall Firmware Package
and extract everything to the one directory.
3. If the ROM is not a Heimdall Firmware Package ...
You can always format from command-line to be sure where the problem is (if any). This will erase your whole /data partition which contains personal data, apps etc.
Boot to TWRP recovery.
Find block device for userdata partition. Usually it's symlinked at /dev/block/by-name/userdata or /dev/block/bootdevice/by-name/userdata. See FINDING BLOCK DEVICE section ...
If you are flashing ROM, Common causes could be
Battery Failure (Please make sure it is charged fully or upto 70%)
Power Off/ Cancelling process in between.
Please search for the same Model ROM to Upgrade to current version.
Also in Settings, search for Software Update option before trying anything else.