Hi city-to-woods,
We appreciate you reaching out to us. We understand that you are experiencing issues installing the Nike Run app.
We’d like to gather additional details that may help us be in a better position to assist you.
Do you have enough device storage on your Apple Watch?
Have you had the chance to try a different Wi-Fi network to download the app?
In the meantime, we recommend that you restart your devices.
Also, we recommend that you check for a software update on both devices.
While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues.
Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Update your Apple Watch
Take care!