Hi pongkhun178,
Thanks for keeping us updated and reaching back out for more help on this.
You mentioned that you no longer have the original iPhone which the Apple Watch was paired with. If the watch is still showing paired to that iPhone, this is why you aren't able to pair with another iPhone.
The article we shared has what you need to do for this issue. We'll give you the steps again:
If you can't pair an Apple Watch with your iPhone - Apple Support
If you don’t have its paired iPhone, erase your Apple Watch
If you can't unpair Apple Watch because you don't have its paired iPhone, you can erase the watch.
When you erase Apple Watch instead of unpairing it, the watch is still protected by Activation Lock. Before you can pair Apple Watch again, enter the Apple ID and password that was used to set it up.
1. Open the Settings app on your watch, then tap General > Reset.
2. Tap Erase All Content and Settings, then tap again to confirm.
3. Wait for the watch to unpair.
4. After Apple Watch unpairs completely, you can pair again. You'll need the Apple ID and password that were used to set up the watch.
Let us know how that goes.
Cheers!