You need either GarageBand '11, version 6.0.5 on High Sierra or GarageBand 10. Did you upgrade to iLife '11, while you still have been running Snow Leopard? The upgrade has been possible in 2010, when iLife '11 came out. If you find it in your Purchase History, you can reinstall it from the App Store and it will run on High Sierra, as Keith Barkley pointed out. But if you did not upgrade to iLife '11 (from the App Store), there is no chance, as you can no longer purchase GarageBand '11 and update it to version 6.0.5, because Apple removed the updater from the Support page. The best version for High Sierra would be the GarageBand 10 version for your system, but you can currently only download for reinstalling. You cannot purchase any GarageBand version for an older system than the current system version macOS 11 Big Sur.
And I doubt, that your Mac can be upgraded to Big Sur: https://support.apple.com/kb/sp833?locale=en_GB
It is really a pity, but only upgrading to the current system version is well supported by Apple since the introduction of the AppStore. If we wait too long before upgrading to a system version, and it has already been replaced by a newer system, we may not be able to get the updated versions of the apps to go with this system version from the App Store. Even switching from GarageBand to Logic Pro X requires now at least macOS 10.15.6 Catalina.
Perhaps you can restore your Snow Leopard system from the last backup you made to an external volume and boot your Mac from the external drive to work with GarageBand.