Flashing the correct img solved the problem:
heimdall flash --RECOVERY TWRP-220.127.116.11_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 ...