I really don't know how to spawn a forcefield in, but what is blocking me is how do I find a character in a script. (not local script)
Tool = script.Parent
local event = game.ReplicatedStorage
local Players = game:GetService("Players")
local character = Players.Character or Players.CharacterAdded:wait()
event.Block.OnServerEvent:connect(function(plr)
local forceField = Instance.new("ForceField")
forceField.Visible = true
forceField.Parent = character
wait(.5)
forceField:remove()
end)

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Note: This is my best guess on spawning in a forcefield