Ok. This is one we hear a lot. here are some possible causes and solutions
1. Make sure the grass collector is fitted properly. This is the most common, and easy to fix, reason for this issue.
2. Make sure the safety switch is connect properly
3. Make sure the sensor is clear of dust and dirt. This can affect starting as if it doesn't engage properly or is interrupted, the mower will cut out.
4. Check you are sitting firmly on the seat as there is also a sensor under there on all ride-ons. You also, generally, need to be above 7ish stone (50ish kg) to operate it! This is a safety measure.