Ensure you are at least on the latest security patch for your current OS version to maintain compatibility with the Apple Store servers.
As of my last update, GarageBand is compatible with macOS. The system requirements can vary depending on the version, but generally, you'll need: garageband 1035 dmg link
Apple is generally helpful for users who can prove legitimate ownership of the hardware. Ensure you are at least on the latest
Apple maintains a portal for older system software (e.g., Xcode, iWork). While GarageBand isn’t always listed, you can try the following steps: iWork). While GarageBand isn’t always listed
If your Mac supports a newer OS, the App Store will automatically try to download the latest version of GarageBand (currently version 10.4.x or higher). You generally cannot install 10.3.5 on these systems without complex workarounds.