I need my script fixed BACKPACK 'data?'

Well, it looks like you are giving the Keycard to the player, when the player join and then breaking the loop when the requirements are met. There is a CharacterAdded event, which fires on character respawn :

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(char)
		repeat wait() until char -- Waits until full character is loaded	

		-- The code
	end)
end)

So, to simplify your code, you can make it a function and execute it on character respawn.

function GiveKeycard(plr)
	local KeyCard = game.ServerStorage["KeyCard"]

	while wait(1) do 
		if plr:WaitForChild("leaderstats"):FindFirstChild("Minutes").Value >= 300 then 
			KeyCard:Clone().Parent = plr:WaitForChild("Backpack") 
			break 
		end 
	end 
end

Now, you can fire the function on PlayerRespawn :

function GiveKeycard(plr)
	local KeyCard = game.ServerStorage["KeyCard"]

	while wait(1) do 
		if plr:WaitForChild("leaderstats"):FindFirstChild("Minutes").Value >= 300 then 
			KeyCard:Clone().Parent = plr:WaitForChild("Backpack") 
			break 
		end 
	end 
end

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(char)
		repeat wait() until char -- Waits until full character is loaded	
		GiveKeyCard(player)
	end)
end)

Lemme know if it helps!