i am making a simulator game but the walk speeds do not match up....?

I have a few answers, try them and see which one fits your preference:

#1. Nguyenlegiahung's answer (improved)

game:GetService("Players").PlayerAdded:Connect(function(plr) 
-- When a Player is Added
	plr.CharacterAdded:connect(function(char) 
-- When a player's character is added
		local hum = char:WaitForChild("Humanoid")
 -- Get the player's humanoid
		pcall(function() 
-- Basically a function that stops errors from stopping the whole script
			local loopc1 = coroutine.wrap(function() 
-- Basically a silent function
				while wait() do
 -- Forever loop
					hum.WalkSpeed = 16 * (Weight.Value / 250) + 1 
-- The equation
				end
			end) loopc1() -- Run the silent function
		end)
	end)
end)

#2. Making sure WalkSpeed isn't 0

game:GetService("Players").PlayerAdded:Connect(function(plr) 
-- When a Player is Added
	plr.CharacterAdded:connect(function(char) 
-- When a player's character is added
		local hum = char:WaitForChild("Humanoid")
 -- Get the player's humanoid
		pcall(function() 
-- Basically a function that stops errors from stopping the whole script
			local loopc1 = coroutine.wrap(function() 
-- Basically a silent function
				while wait() do
 -- Forever loop
					hum.WalkSpeed = (16 * 1) * (Weight.Value / 250)
-- The equation
				end
			end) loopc1() -- Run the silent function
		end)
	end)
end)

#3. Answer #1 and #2 combined

game:GetService("Players").PlayerAdded:Connect(function(plr) 
-- When a Player is Added
	plr.CharacterAdded:connect(function(char) 
-- When a player's character is added
		local hum = char:WaitForChild("Humanoid")
 -- Get the player's humanoid
		pcall(function() 
-- Basically a function that stops errors from stopping the whole script
			local loopc1 = coroutine.wrap(function() 
-- Basically a silent function
				while wait() do
 -- Forever loop
					hum.WalkSpeed = ((16 * 1) * ((Weight.Value / 250) + 1))
-- The equation
				end
			end) loopc1() -- Run the silent function
		end)
	end)
end)