How do I get my animation to play when I click an object and stop when I click it again?

Your script is not working because you're defining the local variable at a lower scope than the needed, you need to set it globally to the script, however you're setting it inside the if what makes it just be defined inside it.

Also, remember your script isn't a local script(i belive) so it would re-write the variable everytime someone clicked the part, so you need to bypass that. In this case I used a table that stores each players animation track, so we can easily stop it

local Flourbag = game.Workspace.Flourbag
local clickDec = game.Workspace.Flourbag.ClickDetector
local Animation = Instance.new("Animation")
Animation.Parent = game.Workspace
Animation.AnimationId = "http://www.roblox.com/asset/?id=5446725528" 
local animTracks = {} -- Here we define a table, that will store the animation for each player

clickDec.MouseClick:Connect(function(Player)
    if not animTracks[Player.Name] then --Here we detect  if player currently don't have a animtrack set...
        animTracks[Player.Name] = Player.Character.Humanoid:LoadAnimation(Animation)
        animTracks[Player.Name]:Play() 
    else
        animTracks[Player.Name]:Stop() --Stop the animation from running
        animTracks[Player.Name] = nil -- Undefine player animtrack, so we can play the animation again
    end
end)

If you want more information about scopes, here it is:

https://developer.roblox.com/en-us/articles/Scope

if you want more information about tables(and dictionaries), here it is

https://developer.roblox.com/en-us/articles/Table

If this answers your question, please mark it as accepted :)