GoSystems Build Failing

Hello, I have installed GoSystems Third Person Character Controller into my Unity Scene. I am using Unity 2021.3.1f. I am attempting to make a Build of the Demo Scene that came with the Third Person package. The Build is failing and I am getting errors like this

Assets\\Go Systems\\GoCharacterController\\script\\GoSystemsEditor.cs(601,14): error CS0246: The type or namespace name \’MenuItemAttribute\’ could not be found (are you missing a using directive or an assembly reference?)

Assets\\Go Systems\\GoCharacterController\\script\\GoSystemsEditor.cs(601,14): error CS0246: The type or namespace name \’MenuItem\’ could not be found (are you missing a using directive or an assembly reference?)

There are about a dozen more \”namespace name could not be found\” errors. I have not edited the original scripts. Any help will be appreciated, Thanks!