Hey Athenaaaa,
Welcome to Apple Support Communities.
We see you're experiencing an issue with the Measure app. Below we've provided some great troubleshooting steps to help with the issue you're experiencing with the app. You do not have to repeat any of the ones you're already going through.
1. Close and reopen the app
Force the app to close. Then open the app to see if it works as expected.
2. Restart your device
Restart your iPhone or restart your iPad. Then open the app to see if it works as expected.
3. Check for updates
To see if updates are available for the app, open the App Store, tap your profile icon, then scroll down to see if updates are available. If updates aren't available for the app, update the software on your iPhone or iPad.
4. Delete the app, then redownload it
Delete the app. When you do this, you might lose data that was stored in the app. Then redownload the app. Then check to see if the problem with the app is fixed.
The steps are also in this article: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open.
If the issue persists, and your iPhone isn't up-to-date, we recommend updating it to the latest version. Updates may include new features, security updates, and fixes. The latest version for the iPhone 12 Pro is 16.3.1.
If the iPhone isn't up-to-date, check out the steps below to back up and update. The steps are available here as well: Update your iPhone or iPad.
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
Hope this helps. Take care.