I suppose fastboot mode was activated. I pressed volume- and power button simultaneously and saw a green android with a hat on the screen, with "fastboot" label underneath.
I am not sure what's going on with sparse "super.img". I followed every steps in the video by u/ingbrzy.
Your help is needed. Keep failing for 1.5 hrs at the following steps, for about 10 times.
Sending sparse super 7/7 failed (error reading sparse file)
fastboot: command failed
fastboot: error device does not support slots
Erasing'metadata'
fastboot: error: command failed
Rebooting
Fastboot...