Right-click the app and select for the first launch (this bypasses some Gatekeeper "unidentified developer" errors if the DMG was packaged manually).
The icon bounced once. Twice.
If possible, compare the file's SHA-256 hash with known clean versions listed on Apple forums. Avoid "Cracked" Sites: garageband 1035 dmg link
If you have never added GarageBand to your Apple ID, the App Store may prevent you from downloading an older version. You can bypass this by briefly signing into your Apple ID on a friend's Mac that already has the latest GarageBand installed, which "adds" the app to your purchase history. 🛠️ Troubleshooting Tips Right-click the app and select for the first