For future reference, be aware that if you use AltStore PAL to distribute your app in Europe, you may receive the below message:

In that case, make sure you have fully signed https://developer.apple.com/contact/request/alternative-eu-terms-addendum/ and it's worth checking if there are other forms that need to be filled for your apps.
You'll need to wait for about 15 minutes, then go to the AltStore settings, and clear cache. After that, you should be able to install your app.
If it didn't resolve the issue, and if you have a Mac, you should be able to plug your device in and examine logs. The same can be achieved via libmobiledevice in the command line for other OS (like Windows).
Set a filter for "MarketplaceKit", or your apps bundle ID, or managedappdistributiond, or altstore.
For future reference, be aware that if you use AltStore PAL to distribute your app in Europe, you may receive the below message:

In that case, make sure you have fully signed https://developer.apple.com/contact/request/alternative-eu-terms-addendum/ and it's worth checking if there are other forms that need to be filled for your apps.
You'll need to wait for about 15 minutes, then go to the AltStore settings, and clear cache. After that, you should be able to install your app.
If it didn't resolve the issue, and if you have a Mac, you should be able to plug your device in and examine logs. The same can be achieved via libmobiledevice in the command line for other OS (like Windows).
Set a filter for "MarketplaceKit", or your apps bundle ID, or managedappdistributiond, or altstore.