"Remix FX" isn't available on your system

I have this exact same issue as well. I created a whole song on my iOS device, imported to the Mac and more than half the sounds are missing. Even after downloading all the additional files on the mac by clicking on the menu and selecting "Restore Sound Library" I still get the "Remix FX isn't available on your system" error message when trying to open that specific song.

Ok, after tinkering around with this issue for a while I've come to the conclusion that the desktop version of Garageband (version 10.1) isn't yet equipped to support all of the effects and sounds from the latest iOS version (2.1). Currently, your only option is to open the project up in Logic Pro X. "Remix FX" is available in Logic Pro X, as well as all of the new sounds from the iOS version of Garageband (2.1) and the project opened up seamlessly without any errors. I tried this with an earlier version of Logic Pro and it didn't work, so at the time of this writing, Logic Pro X is your only desktop solution for supporting all of the sounds and effects from Garageband 2.1. I hope Apple releases a free update to the desktop version of Garageband for those who don't have Logic Pro X, otherwise they can't really tout Garageband as a multi-platform application.

I'm getting the same error for music memos created on the iPad with the Music Memos application. The Remix FX instruments are not yet supported in GarageBand 10.1.

Install the LogicPro X update made it worse. After I installed this update, even more instruments in GarageBand stopped working, see this discussion:

Some synths now no longer are working in GarageBand, because Logic replaced them by newer versions, that are not working in GarageBand 10.1.

I tried to extract the loops manually from the GarageBand iOS application, but does not work for the FX instruments.

We have to wait for an update.

To get the loops to your Mac, however, see the solution by mprenfro

How to get new iOS GB loops into your Mac

Man, what a mess.

I'm wondering what the **** are they doing releasing a multiplatform application that, in fact, is not working as it should.

****** pretty bad.

It is a two-sided sword.

The problem is, that Apple is recently developing the software for the mobile devices faster than for the macs, because they are releasing new iPhones and iPads every year. And these new models require new versions of iOS and updates to some applications. Whenever a new iPhone or iPad is about to hit the market, we will see a new version of iOS and updates for some apps. It has been this way since iOS 7 and the introduction of iCloud. It is necessary to release the new iOS versions, because the new models require them and would not work at all otherwise. The upgrades for the mac will follow a bit later. For example, we had to wait several month for a Photos for Mac.app and iCloud Photo Library on the Mac, while it has been available on the iOS devices since the fist release of iOS8. New iPhone and iPad models will be released, as soon as they are ready to go, whether the software counterparts on the Macs are ready or not. The developement of the Mac versions simply takes longer, because they are more versatile. If you are using both a Mac and an iOS device it is usually better to wait with the update for the mobile devices until the Mac counterparts are ready as well.

Just click on the instrument then click "Show in Tracks View" and then start recording

léonie, does this also mean that certain EXS sounds from a session in GarageBand 5 (or around this iteration) will not be recognised by Logic X?

I found this to be the case when opening those earlier GarageBand sessions in GarageBand 10 and they were not available. So, I have bought Logic X and am downloading the Additional Sound Library Files in the hope that some of those software sounds (drum kits, brass, etc) will be available.

I don't understand Apple's 'destructive' updating of, say, a synthetic trumpet with a more real-sounding trumpet. Why not let them co-exist separately - because, who is to say that one is better than the other? Users will have laboured for a long time contextualising certain sounds only to find that they are now missing!

léonie, does this also mean that certain EXS sounds from a session in GarageBand 5 (or around this iteration) will not be recognised by Logic X?

It will probaby depend on how you upgraded to newer versions. I always left the instruments and loops from earlier GarageBand versions installed and they continued to work in the new versions. But if you clean-installed when upgrading, sounds may be missing in Logic and GarageBand.

BTW., after the update to the newest GarageBand version 10.1.1, I do not get any longer the error message "Remix FX" isn't available on your system", when I am opening a GarageBand iOS song. Something has been fixed.