How can I insert a 'body force' into a player and change its variables?

I want to make a game where one of the mechanics is you press a or d for to slide left or right. Currently I only have it so a variable "lanes" increases of decreases when you press "d" or "a", it doesn't move the character. my first idea is to move the player using body forces, if there is anything else i can do to do something similar tell me. later, i want to animate the slides, but i want to finish game mechanics.

local lanes = 0



--slide right
function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.D then
		print("D was pressed")
		lanes = lanes + 1
		print(lanes)
		print("--------------")
	end
end
 
--calls slide right
game:GetService("UserInputService").InputBegan:connect(onKeyPress)


--slide left
function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.A then
		print("A was pressed")
		lanes = lanes - 1
		print(lanes)
		print("--------------")
	end
end
 
--calls slide left
game:GetService("UserInputService").InputBegan:connect(onKeyPress)

--overflow control keeps lanes between -2 and 2. (doesn't work yet)
while lanes == 3 do
	lanes = lanes - 1
end

while lanes == -3 do
	lanes = lanes + 1
end

Lets make this work with filtering enabled, We will use your script you made to decide the lanes value, then we will send those over to the server through a remoteevent, multiplying it by the variable Mul(default is 200) then set the body force's X to that, it will create a body force when the local script loads, resetting will not cause a problem for it will redo it.

place a RemoteEvent inside workspace called "Lane"

LocalScript inside StarterPack

workspace:WaitForChild(game.Players.LocalPlayer.Name)
workspace:WaitForChild("Lane"):FireServer(0,true)

local lanes = 0



--slide right
function onKeyPress(inputObject, gameProcessedEvent)
    if inputObject.KeyCode == Enum.KeyCode.D then
        print("D was pressed")
        lanes = lanes + 1
        print(lanes)
        print("--------------")
	workspace.Lane:FireServer(lanes)
    end
end

--calls slide right
game:GetService("UserInputService").InputBegan:connect(onKeyPress)


--slide left
function onKeyPress(inputObject, gameProcessedEvent)
    if inputObject.KeyCode == Enum.KeyCode.A then
        print("A was pressed")
        lanes = lanes - 1
        print(lanes)
        print("--------------")
	workspace.Lane:FireServer(lanes)
    end
end

--calls slide left
game:GetService("UserInputService").InputBegan:connect(onKeyPress)

--overflow control keeps lanes between -2 and 2. (doesn't work yet)
while lanes == 3 do
    lanes = lanes - 1
end

while lanes == -3 do
    lanes = lanes + 1
end

Regular script inside workspace

local rem=workspace:WaitForChild("Lane")
local mul = 200
rem.OnServerEvent:connect(function(player,lane,it)
	if it~=nil and it==true then
		if workspace:FindFirstChild(player.Name)~=nil then
			if workspace:FindFirstChild(player.Name):FindFirstChild("UpperTorso")~=nil then
				Instance.new("BodyForce",workspace:FindFirstChild(player.Name).UpperTorso).Force=Vector3.new(0,0,0)
			elseif workspace:FindFirstChild(player.Name):FindFirstChild("Torso")~=nil then
				Instance.new("BodyForce",workspace:FindFirstChild(player.Name).Torso).Force=Vector3.new(0,0,0)
			end
		end
	elseif it==nil then
		if workspace:FindFirstChild(player.Name)~=nil then
			if workspace:FindFirstChild(player.Name):FindFirstChild("UpperTorso")~=nil then
				if workspace:FindFirstChild(player.Name).UpperTorso:FindFirstChild("BodyForce")~=nil then
					local t = workspace:FindFirstChild(player.Name).UpperTorso
					workspace:FindFirstChild(player.Name).UpperTorso.BodyForce.Force=Vector3.new(((t.CFrame.rightVector.X)*lane)*mul,0,((t.CFrame.rightVector.Z)*lane)*mul)
				end
			elseif workspace:FindFirstChild(player.Name):FindFirstChild("Torso")~=nil then
				if workspace:FindFirstChild(player.Name).Torso:FindFirstChild("BodyForce")~=nil then
					local t = workspace:FindFirstChild(player.Name).Torso
					workspace:FindFirstChild(player.Name).Torso.BodyForce.Force=Vector3.new(((t.CFrame.rightVector.X)*lane)*mul,0,((t.CFrame.rightVector.Z)*lane)*mul)
				end
			end
		end
	end
end)

Hoped this helped if you have any questions please do ask :)