Hi
The built-in cellular feature is only available on Apple Watch Series 3 (GPS + Cellular) models. It is not available on Apple Watch Series 3 (GPS):
- Apple Watch - Compare - Apple
- Apple Watch Series 3 - Apple
- Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support
To check your Apple Watch model - along with its service and support coverage - enter the serial number here:
- Check Your Service and Support Coverage - Apple Support
- Find the serial number for your Apple Watch - Apple Support
- Identify your Apple Watch - Apple Support
If you have bought the wrong model by accident, then, subject to the seller's return policy, you may wish to consider returning it for a refund / exchange. For products that have been purchased directly from Apple, Apple's return policy in the US, for example, is detailed here, along with instructions: