This is most likely the reason for my efivars error.
I did follow @glathe advice and used one of his extended ISO images with Vivobook S15 as an “alias” for my laptop. It did boot and I was able to Try Ubuntu.
I didn’t connect to a network, screen looked as expected, used the laptop keyboard and pad.
I tried a few apps like Files and Terminal.
The fan cooler was active all the time, so the Power management didn’t seem to be working properly.