Where do I go from Cerulean City? [closed]
You need to head south on route 5 and take the underground passage to route 6 and Vermilion City. There is a gym in this city which is your next stop.
Not sure what is going on with the Bike voucher, I would try depositing/selling a few TM you are not using in the PC and try again. In the original games you have limited space in your bag.