The mod is trying to access non-existent code while adding the tool to the character's inventory. The game's item management logic was changed at some point during development, and your game version probably predates that.
I'm afraid that updating your game is likely your only real option in this case, unless you're willing to tinker with the code in this mod to get it to use the old process. If you are, I'd suggest consulting the "useitem" function in the "InventoryListModule" as it shows how gear is equipped on characters.
If you choose to update your game instead, be aware that Conquest Tweaks is not fully compatible with the modern version of Strive: Conquest. You will have to disable some of its modules.
My apologies that I could not help you further.