So what you did was completely reasonable, Edit, but this is our recommended install method.
I would be very wary of reflashing as there is no “factory” image. There is an image, linked in our docs, but l suggest you delay that approach.
(seems) my guess, to me to be in your flashtool and its relationship with Lubuntu. I am not at a PC to double check this but in Debian I required a very recent kernel, which I did not try very hard to acquire.
Perhaps you could link your source for flashtool? On reflection, let’s avoid that.
Edit. Do you have access to Bootloader?
Reading the link above, are you able to complete all the prerequisites? Please don’t rush!
Then perhaps the fastboot unlock method will work.