Aeursemb wrote:
However, it is possible to install it from the Mac App Store by going to the Purchased section of your user profile and finding Garageband amongst those apps. Trying to download from there will yield a message that allows you to choose the latest supported version of Garageband to be installed instead of the most recent one.
While this method should always work in theory, in practice at the moment there seems to be generally something faulty in the App Store land:
In the past two days I was updating a Mojave partition which I haven't used in weeks; but just a few weeks ago I have successfully downloaded the latest Mojave compatible version of Logic Pro (10.5.1). A few days ago I wanted to update some other apps from the App Store, but got an error everytime. Waited a day, logged out of App Store, logged in again, and the update downloads have worked again.
Today I noticed that I never downloaded GarageBand to this partition yet. So I did the usual procedure by clicking the "download-from-cloud" button, only to get the dreaded "GarageBand can't be installed on XY because macOS version 11 or later is required" message. App Store log-outs and Mac reboots didn't help, except once when the "you can download the latest compatible version" briefly appeared, only to be replaced by "can't be installed" message again.
Luckily the latest Catalina compatible version of GarageBand is also 10.3.5, so in the end I have copied the app from my Catalina partition.
For what it's worth, on Catalina I also see the "Update" button for the Big Sur version of GarageBand, which of course doesn't work: no Big Sur on my MacBook Pro 2012, the last model with matte display and DVD drive which they will once pry from my cold, dead hands… ;)
Long story short:
Folks, don't waste too much of your time trying to fix it yourselves since there likely not much that can be fixed at user's end. We're at Apple's mercy here.