It starts then stalls immediately

The engine catches and runs for a second or two, then dies, and the pattern repeats on every attempt. This signature is very telling: the engine starts on the fuel already present in the carburetor's float bowl (or an injection priming charge), but something is preventing it from getting fuel or being allowed to keep running once that reserve is used up. So you're looking for what 'cuts it off' right after starting. The main leads center around fuel supply and safety systems. On the fuel side: clogged jets or a clogged idle circuit (the engine runs on the float bowl reserve then stalls from lack of continuous fuel supply), a dead vacuum-operated fuel tap (that no longer opens to let fuel through), a blocked tank vent (a vacuum builds up and cuts off fuel flow), or a mismanaged choke (left open, or not used when cold). On the electronic side, on newer bikes: an immobilizer/security system that doesn't recognize the key allows a brief start then cuts the ignition, with a key/lock warning light on. Two quick tests strongly guide the diagnosis: loosening the tank cap (if it no longer stalls, it's the vent) and holding a bit of throttle (if the engine keeps running as long as you give it gas, it's more likely an idle-circuit fuel issue or the choke; if it stalls anyway, it's more likely the fuel tap or the immobilizer). Urgency: no immediate danger, but an unusable bike; start with the free checks (cap, choke) before any disassembly.