Some of the Jam Pack installers are using still old PowerPC code and cannot run on Big Sur. I took the easy way out and purchased MainStage from the App Store. It includes all five Jam Packs and installs them in the same location as needed for Logic Pro X and GarageBand. MainStage is cheaper than a single JamPack, so it is quite a bargain.
Or, if you still have a Time Machine backup of your Mac from before the upgrade, with all Jam Packs installed, you could simply restore them from your backup.
The instruments are installed in /Library/Application Support/GarageBand/Instrument Library
and the loops in /Library/Audio/Apple Loops
Try to restore these folders from your backup.