Why my apple watch dosent support esim?

Talk to your Pakistan cellular providers.

Until then, you can use a GPS Only Apple Watch and do everything the cellular model does, as long as your iPhone is nearby.

For the Esim to work you must have a network provider that supports the apple cellular watch,here is a list of carriers that support the watch;

Apple Watch - Carriers - Apple

Because there are no network providers that support the watch yet.