Gen 8 is not avaiable anymore....

the panel is still available but it is just changed its name to match other mod settings.
the migration of the name is done on 1.1.9 to 1.2.0

I have the same problem. The recipe exists in the technology tree, but it is never visible on the assemblers.

i cannot sure what is the actual issue

we can research it, but we can't build the Gen8 panel

you can do something like
/sc game.player.force.technologies['compound-energy-7'].researched=false
/sc game.player.force.technologies['compound-energy-7'].researched=true
/sc game.player.force.recipes['solar-panel-8'].enabled=true
on the console to reload the technology or the recipe

the reset of the technologies worked. thx