I cannot connected my new iPhone 13 to my watch

Did you unpair the watch from your old phone first;

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

Then;

Setup your Watch to a new iPhone

To pair your Watch you need to make sure it is restored and disconnected from your previous iPhone. If it's not you can erase you Watch via the iPhone to which it is connected or, if you don't have it, directly via the Apple Watch.

To do so, in either cases, you can follow this article Unpair and erase your Apple Watch - Apple Support

After the restoring process you can pair your Watch by simply hold it near your new iPhone and following the on-screen instructions. For more information follow this other article Set up your Apple Watch - Apple Support