Ios 5.3. Os 12.4 installed. Still no walkie talkie app
Apple Watch
All parties must have the latest updates and have FaceTime activated. Try it again, the App should show up on your iPhone Watch App.
In which country are you using the Apple Watch? The Walkie-Talkie app isn't available in all countries or regions: https://www.apple.com/watchos/feature-availability/#apps-walkie-talkie
Maybe this is the reason you don't see it.
Anyway, here're further infos about Walkie-Talkie: https://support.apple.com/en-us/HT208917
U.S. San diego
I've read that some users with deleted FaceTime app on iPhone didn't get Walkie-Talkie app on their Apple Watch.
It seems that Walkie-Talkie is dependent on FaceTime to work.
Other than that, if you've just updated iOS/WatchOS, I'd simply try a restart of both devices (if you haven't already done, of course).
Walkie talkie still has not uploaded to apple watch after latest updates
