It feels like we skipped something, or am I dumb and dont remember and have to reread?
Thanks for the translation!
You didn't skip anything. Having just done a reread myself the story flows normally.
In previous chapter the owner of that shop said the transformation device can't be used by other people beside the owner so that reason not gonna work
The completed version yes. But there is a component(core) to the device. It's likely that component was mixed up or miscalculated somehow while the device was being made.
Anyways, nice to see this series be picked up. Stories like this are far more interesting than normal ones, as it is neat to see the MC learns to be a real hero, rather than just his ideal version. It's kinda like a relative to the "wants cheat powers but gets crappy one, and must learn to make due with what he got" genre.