Hey there Cl29s,
We understand you're unable to use the Watch app on your iPhone. We'll be happy to help.
First, you can locate any app you have installed by using the App Library feature. This is explained in full detail here: Organize the Home Screen and App Library on your iPhone
Once you find the app, see if it opens. If not, let's disable any restrictions to see if that might be causing issues. Go to: Settings > Screen Time > Content & Privacy Restrictions
For full details on restrictions, check this out: Use parental controls on your child's iPhone, iPad, and iPod touch
If you still experience issues, try searching for "Watch" in the App Store. What happens when you tap Open there? Does it give any error messages?
Cheers!