If you have never downloaded GarageBand with your current Apple ID, the App Store will simply block you from downloading it on Catalina. You can bypass this restriction by temporarily using a newer Mac.
On the first launch, it may ask to download additional content and "Essential Sounds".
You are trying to run 32-bit VST/AU plugins (Catalina dropped 32-bit support entirely). Fix: Only use 64-bit Audio Units. Run your plugins through a validator like AU Lab or update to the 64-bit version of the plugin.
If the App Store refuses to download the legacy version, try signing out of the App Store, restarting your Mac, and signing back in. Missing Sound Library
Apple ensures that even if you are not on the latest OS, you can still download the last compatible version. Do pay for third-party "downloaders"—GarageBand is free from Apple. Garageband For Catalina 10.15.7 -FREE- Download
Search for GarageBand and click "Get" or "Download." This links the app to your Apple ID history.
Download gigabytes of free Apple Loops, synth pads, acoustic instruments, and drum kits.
After installation, open GarageBand. The first time you run it, you will be prompted to download the full (additional loops, sounds, and instruments). This is also free , though it requires extra disk space (~10–15 GB).
"GarageBand cannot be installed because macOS version X or later is required" If you have never downloaded GarageBand with your
Access to thousands of loops, instruments, and "Drummers."
If you have never downloaded GarageBand on your Apple ID before, you can use a secondary Mac running a newer macOS version to add the app to your account history.
October 2023 Target OS: macOS Catalina (Version 10.15.7) App Status: Free
Scroll through your purchased apps list until you find GarageBand. Step 4: Click the Cloud Download Icon You are trying to run 32-bit VST/AU plugins
Go to → find GarageBand → download the last compatible version for Catalina.
Ensure your System Clock is set correctly, as mismatched dates can cause "damaged" installer errors on older macOS versions. Conclusion
Once downloaded, GarageBand will open normally on macOS Catalina 10.15.7.
By downloading the Catalina-compatible version (usually version 10.3.x), you are getting a very powerful Digital Audio Workstation (DAW) for free.