Hi, to get the last achievement on Steam you need to play through all possible routes. At the bottom there is a 'skip' button you can use to quickly skip any text you have read before.
If you played the itch version before the Steam version then go into 'Settings' and click 'Sync Achievements', this will update Steam with any achievements you will have got.
Please let me know if you still can't get the achievement and I can make a diagram of all the routes.