When you are typing in keywords/variables with underscores, surround it with the backwards apostrophe character. For example: weapon_ui.text = weapon_data["Name"] + ": " + weapon_data["Ammo"] + "/" + "ExtraAmmo"
Open up his part 3 project and then scroll down the script side-by-side with your project to see what differences there are. If you can’t find anything, try pasting his entire code into yours and see if it actually works with your version.