Why can’t I see the Apple Watch icon on my iPhone 8 Plus?

if you mean the watch app then it can be deleted from the phone

if you have then you can read about restoring it here Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

then it's likely in some folder somewhere or the folder layout gotten corrupt and does not display it you can try

to reset the home screen layout

If I reset home screen layout what will t… - Apple Community

and see if it does not end up on one of the home screens then

what happens if you use search to look for it?

if you are unsure how then read

Use Search on your iPhone, iPad, or iPod touch - Apple Support