I can't set the value why?

--|| This is the script its erroring at the line after "Found a seat" ||--
local Prompt = script.Parent
local ActionText = "Hold F to join the car"
local HoldDuration = .8
local MaxActivationDistance = 7.5

Prompt.ActionText = ActionText
Prompt.ClickablePrompt = true
Prompt.HoldDuration = HoldDuration
Prompt.KeyboardKeyCode = Enum.KeyCode.F
Prompt.MaxActivationDistance = MaxActivationDistance
Prompt.ObjectText = "Join the car"
Prompt.RequiresLineOfSight = true

local BackSeats = script.Parent.Parent["Back Seats"]

Prompt.Triggered:Connect(function(Player)
	local Character = Player.Character
	if Character then
		for _, seat in pairs(BackSeats:GetChildren()) do
			if seat.Occupant == nil then
				print("Found a seat")
				seat.Occupant = Character.Humanoid
				break
			end
		end
	end
end)

Error: Can't set the value

https://developer.roblox.com/en-us/api-reference/property/Seat/Occupant This property can only be read from. Attempting to write to it will cause an error.

Like yx10055 correctly said, you cannot set the seat occupant. Instead replace lines 21-25 with the following code.

if seat.Occupant then return end --Make sure seat is empty

if not Character then return end --Check for character

local humanoid = Character:FindFirstChild("Humanoid")

if not humanoid then return end --Check for humanoid

seat:Sit(humanoid) --Player sits if seat is empty, character exists, and humanoid exists