Why would white smoke come out of my tailpipe after changing the valve cover gasket on my 95 Nissan 200sx it only does it on highways and has also started missing never had these problems b4?

White smoke from the exhaust is a symptom of coolant entering the combustion chamber. Most often caused by a cracked or warped head or blown head gasket. Stop driving this vehicle until you have this repaired or you will destroy the engine.