Error "attempt to call a nil value" on the client when calling the module

[ruckuhs]

you could always pass through the name (a string value) of the function you wish to run from the client to the server and hold all functions inside of a table on the Server (or a Server-Side module)…

SERVER SCRIPT (basic example)

local remote = game.ReplicatedStorage.RemoteEvent
local part = workspace.Part

local functions = {
	
	ChangePartColor = function()
		part.BrickColor = BrickColor.random()
	end,
	
	ChangePartShape = function()
		part.Shape = Enum.PartType.Ball
	end,
	
	ChangePartSize = function()
		local x = math.random(0, 20)
		local y = math.random(0, 20)
		local z = math.random(0, 20)
		
		part.Size = Vector3.new(x, y, z)
	end,
}

remote.OnServerEvent:Connect(function(player, action)
	functions[action]() -- we find the "action" name inside of the functions table and run it from here as long as it exists
end)

LOCAL SCRIPT

local remote = game.ReplicatedStorage.RemoteEvent

remote:FireServer("ChangePartSize") -- make sure this matches the name of the function inside of your "functions" table in the Server Script
remote:FireServer("ChangePartColor")
remote:FireServer("ChangePartShape")