The bike voucher is an item used in Pokemon Gold, Silver, and Crystal to obtain a bike from the bike shop in Goldenrod City. FireRed does not have bike vouchers or a bike shop, and bikes are obtained differently.
In FireRed, you get a bike from the Cycling Road. This is a route located just north of the town of Fuchsia City. You can't get a bike until after you have obtained the HM02 (Fly) from the Pokemon Fan Club in Goldenrod City.
Once you've obtained the HM02, the Cycling Road will be accessible. There's a little cutscene, and after talking to the person there, you'll be given a Bicycle.
Remember, there are no bike vouchers in FireRed, and you obtain a bike by completing the Cycling Road.