Hello I'm having trouble trying to connect my Apple Watch to my new iPhone

Make sure you unpaired your watch from your old phone to do this, open the Apple Watch app on your old iPhone go to > My Watch tab > All Watches > Tap the info button next to the watch that you want to unpair > Tap Unpair Apple Watch. ( Keep your Apple Watch and iPhone close together as you unpair them ). If you don't have your old phone anymore, On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. This will reset your Apple Watch to the factory settings.

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 see this page;

How to pair your Apple Watch with a new iPhone

Have you removed/unpaired it from your old phone first?,a watch can only be paired to one phone at a time.