我需要在我的代码中使用“ruby2d”，并编写一个简单的程序来构建我的snake游戏。我试图让程序在屏幕上弹出一个窗口。这是我的密码

require 'ruby2d'

set background: 'navy'

# width = 640 / 20 =32

# height = 480 / 2- = 24

GRID_SIZE = 20

class Snake
    def init
        @positions = [[2,0], [2,1], [2,2], [2,3]]
    end

    def draw
        @positions.each do |position|
            Square.new(x:position[0] * GRID_SIZE, y: position[1] * GRID_SIZE, size: GRID_SIZE, color: 'white')
        end
    end
end

snake = Snake.new
snake.draw
show

我还在终端上运行了gem install ruby2d命令，它给了我这个错误，

Fetching ruby2d-0.10.0.gem
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.6.0 directory.