What ended up working for me was going back to certificates and identifiers page in developer.apple.com and going to the app id, there are TWO tabs to check WeatherKit on.
My mistake was not checking WeatherKit on both Capabilities AND the App Services page. I had only checked it under Capabilities. I was able to get WeatherKit running in 3 different apps after checking both boxes.
LittleBigWig
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Same here, I followed all the instructions and waited 24hrs but I still have not had the sample app successfully load any weather data :(