Unable to pair my Apple Watch Series 5 to my iPhone

Start with the troubleshooting guide linked by Ingo2711 earlier in this thread, and also check any Apple help docs linked from the base guide. Then if you are still unable please post back with more details of what you actually do (actions) and the result you see on the various displays.

Which iOS is installed on your iPhone?

Did you already have a look at the general troubleshooting mentioned in this article?

If you can't pair an Apple Watch with your iPhone - Apple Support

Same.

iOS 17.4.1

How can i fix it?