Hi @Globale welcome to the /e/ forum. You do not quite describe at what level the phone is totally unusable 
The first thing to check is whether you can enter Recovery mode. Before doing anything you want to establish is TWRP was successfully installed or whether you have Android recovery. If Easy installer got to the last stage, did you already boot into TWRP?
One possible reason for this message is the possibility a Samsung or Google account was still on the phone from before you started this.