Inverse Kinematics Weapon System in Unity
Setting up the ability to use IK rigging in unity requires the use of a preview package, at least at the time of writing this 25/06/2021
Add the Animation Rigging package to unity through the package manager, if you cannot find it, it may still be in preview build which will require you to enable preview packages.
To enable preview packages in unity go into to project settings through the edit menu or click on the cog in the package manager and select advanced project settings.
Then in Project Settings find the Package Manager submenu, then at the bottom Advanced Settings, Enable Preview Packages.