The service reminder appearing before reaching 500km (the mileage for the 1st service) on your BS6 Classic 350cc could be due to a couple of reasons:
Pre-set Service Reminder: Some motorcycles come with pre-set service reminders based on a fixed duration rather than mileage. It's possible that the service reminder on your bike is triggered by a specific time interval rather than the actual mileage covered. In this case, it is normal for the reminder to appear before reaching the recommended mileage for the 1st service.
Technical Issue: There could be a technical issue or a malfunction in the service reminder system of your motorcycle. It's worth checking with an authorized Royal Enfield service center to ensure there are no underlying problems.
To address this issue, it is advisable to contact your nearest Royal Enfield service center or consult your bike's owner manual. They will have the expertise to diagnose the problem accurately and provide the necessary assistance. It's important to adhere to the recommended service intervals for proper maintenance and to ensure the longevity of your motorcycle.