Zelle is actually a service that is often integrated into existing banking and payment apps, rather than a standalone app. This means that you may not be able to find it on the App Store as a separate download. Instead, you can try checking your bank's mobile app to see if Zelle is available as a feature.
Another possible reason you can't find Zelle on the App Store is that it may not be available in your region. Zelle is primarily used in the United States, so if you're located outside of the US, you may not be able to access it. Make sure to check the App Store's terms and conditions to see if Zelle is available in your area.