Your frame serial number is stamped into a certain place in the frame. Unfortunately the position can change between models and production batches.
Frame numbers starts either with “YPK…” or just “S…” (Infinity). All other number you might find on the bikes are NO frame numbers.
Please check the following positions:
1. Headset: the tube below the handlebars, front side, below the logo.
2. Bottom bracket: the bottom bracket is the frame part where the axle of the crank (pedals) go through the frame. It’s a round tube, the number is right at the bottom. Best to get there is to lay the bike on it’s right side (on a soft surface).