If GarageBand will no longer open on your iPhone or iPad after you have updated to iOS 11, you’re not alone. Thankfully, Apple is aware of the issue and is “investigating solutions.”
In the meantime, the company has put out a support document offering a workaround fix that requires turning off iCloud syncing for GarageBand on your device.
1) Open the Settings app on your iPhone or iPad.
2) Tap your name or profile picture at the top.
3) Tap iCloud.
4) Turn off GarageBand underneath the heading Apps Using iCloud.
This will allow you to launch the app, but you won’t be able to access any GarageBand songs stored in iCloud Drive until Apple squashes this bug.
Flipping that switch won’t remove any GarageBand projects saved in iCloud. New songs you create will be saved to your device, not iCloud.
We’ll inform you about availability of the promised fix as soon as it becomes available.