Why am i getting an error for this explosion spell?

classic mistake. It says Position is not a valid member of model Dummy. This means you cannot position whole models. Instead, do this.

local Part = script.Parent.HumanoidRootPart

local ex = Instance.new("Explosion")
ex.ExplosionType = Enum.ExplosionType.NoCraters
ex.Position = Part.Position
ex.BlastPressure = 0
ex.BlastRadius = 0
ex.Parent = Part
for _, v in pairs(game.Players:GetPlayers()) do
    local chr = game.Workspace:FindFirstChild(""..v.Name)
    if (Part.Position - chr.HumanoidRootPart.Position).magnitude <= 12.5 then
        coroutine.wrap(function()
            chr.Humanoid:TakeDamage(40)
            local humanoid = v.Character.Humanoid
            local torso = chr.HumanoidRootPart

            humanoid.PlatformStand=true
            torso.CFrame=torso.CFrame*CFrame.new(0,0.2,0)*CFrame.Angles(-.5,0,math.random(0,0))
            torso.Velocity=torso.CFrame.lookVector*-150
            wait(3)
            humanoid.PlatformStand=false
        end)();
    end
end

wait()
script:Destroy()

so basically, position is not a model's property. It is only a property of baseparts, negativeparts, meshparts, unions and explosions, not whole models. It's annoying but you'll get used to it. Basically what I did is change the first line from the dummy to the dummy's humanoidrootpart (the center of the dummy)